Better error reporting when a config key in template is Unknown

This patch will allow to fix the following bug much faster:

```
1 error(s) occurred:

* Unknown configuration key: output_directory
```

Related configuration:

```
"output_directory ": "build/sl_base/",
```

After the patch, the error reporting will be:

```
1 error(s) occurred:

* Unknown configuration key: "output_directory¤"
```

Address go vet reports

Fixes the following vet reports:

provisioner/salt-masterless/provisioner.go:119: arg cmd for printf verb %s of wrong type: *github.com/mitchellh/packer/packer.RemoteCmd
provisioner/salt-masterless/provisioner.go:121: arg err for printf verb %d of wrong type: error
provisioner/salt-masterless/provisioner.go:142: arg err for printf verb %d of wrong type: error
provisioner/salt-masterless/provisioner.go:157: arg err for printf verb %d of wrong type: error
provisioner/salt-masterless/provisioner.go:172: arg err for printf verb %d of wrong type: error
provisioner/salt-masterless/provisioner.go:216: arg err for printf verb %d of wrong type: error

Fixes the following vet report:

provisioner/puppet-masterless/provisioner.go:196: missing argument for Errorf(%d): format reads arg 1, have only 0 args

Fixes the following vet reports:

post-processor/vagrant-cloud/client.go:81: arg resp for printf verb %s of wrong type: *net/http.Response
post-processor/vagrant-cloud/client.go:99: arg resp for printf verb %s of wrong type: *net/http.Response
post-processor/vagrant-cloud/client.go:131: arg resp for printf verb %s of wrong type: *net/http.Response
post-processor/vagrant-cloud/client.go:156: arg resp for printf verb %s of wrong type: *net/http.Response
post-processor/vagrant-cloud/client.go:175: arg resp for printf verb %s of wrong type: *net/http.Response

Fixes the following vet report:

post-processor/vagrant/post-processor_test.go:135: possible formatting directive in Fatal call

Fixes the following vet reports:

packer/template_test.go:496: possible formatting directive in Fatal call
packer/template_test.go:528: possible formatting directive in Fatal call

Fixes the following vet report:

communicator/ssh/communicator_test.go:88: range variable newChannel captured by func literal

Fixes the following vet reports:

common/step_create_floppy_test.go:79: possible formatting directive in Fatal call
common/step_create_floppy_test.go:89: possible formatting directive in Fatal call
common/step_create_floppy_test.go:180: possible formatting directive in Fatal call
common/step_create_floppy_test.go:190: possible formatting directive in Fatal call

Fixes the following vet report:

builder/vmware/iso/builder_test.go:178: arg b.config.DiskSize for printf verb %s of wrong type: uint

Fixes the following vet report:

builder/vmware/common/step_shutdown_test.go:130: missing argument for Fatalf("%s"): format reads arg 1, have only 0 args

Fixes the following vet report:

builder/virtualbox/iso/builder_test.go:86: arg b.config.DiskSize for printf verb %s of wrong type: uint

Fixes the following vet report:

builder/qemu/builder_test.go:163: arg b.config.DiskSize for printf verb %s of wrong type: uint

Fixes the following vet report:

builder/parallels/iso/builder_test.go:78: arg b.config.DiskSize for printf verb %s of wrong type: uint

Fixes the following vet reports:

builder/openstack/artifact.go:44: arg a.ImageId for printf verb %d of wrong type: string
builder/openstack/step_wait_for_rackconnect.go:24: arg server for printf verb %s of wrong type: *github.com/mitchellh/gophercloud-fork-40444fb.Server

Fixes the following vet reports:

builder/googlecompute/step_create_instance_test.go:42: possible formatting directive in Fatal call
builder/googlecompute/step_teardown_instance_test.go:29: possible formatting directive in Fatal call
builder/googlecompute/step_teardown_instance_test.go:39: possible formatting directive in Fatal call

Fixes the following vet reports:

builder/digitalocean/builder_test.go:267: arg b.config.SSHUsername for printf verb %d of wrong type: string
builder/digitalocean/builder_test.go:300: arg b.config.RawSSHTimeout for printf verb %d of wrong type: string
builder/digitalocean/builder_test.go:341: arg b.config.RawStateTimeout for printf verb %d of wrong type: string
builder/digitalocean/builder_test.go:382: arg b.config.PrivateNetworking for printf verb %s of wrong type: bool
builder/digitalocean/builder_test.go:397: arg b.config.PrivateNetworking for printf verb %s of wrong type: bool

Fixes the following vet report:

builder/amazon/ebs/step_create_ami.go:90: arg resp.Return for printf verb %s of wrong type: bool

Fixes the following vet reports:

builder/amazon/common/step_ami_region_copy.go:81: arg target for printf verb %s of wrong type: github.com/mitchellh/goamz/aws.Region
builder/amazon/common/step_ami_region_copy.go:93: arg target for printf verb %s of wrong type: github.com/mitchellh/goamz/aws.Region

docs/templates: add note about escaping

post-processor/vagrant: fix test failure on Windows

Add Rackconnect support to openstack builder

ec2-upload-bundle uses deprecated parameter

Tell people to use OVF v2

Updated doc with config

builder/digitalocean: fix response decoding after api changes

[skip ci]

website: address spelling mistakes

The example error message being in here is important so people can search for that error message and find the answer easily. The default in my VirtualBox is V1, so many people will run into this.

This adds two config options that we need in order to successfully build
our Rackspace images.

First, a boolean `rackconnect_wait` option which waits for the
RackConnect metadata to appear.

Second, an `ssh_interface` option, for rackconnect users who have more
prohibitive firewalls on the 'public' interface and want to ensure all
traffic to the server goes over the 'private' one.

Finishes #952.