core: Replace Gob RPC with MsgPack RPC

Conflicts:
	builder/amazon/common/ssh.go
	builder/digitalocean/ssh.go
	builder/googlecompute/ssh.go
	builder/openstack/ssh.go
	communicator/ssh/communicator_test.go
	communicator/ssh/keychain.go
	communicator/ssh/keychain_test.go

Fix unknown command and syntax error of the ESX5Driver, see #1012, #1015

provisioner/chef-client: validation_client_name

builder/googlecompute: configure instance name

Retain the current default of an instance name generated by UUID, but allow
users to choose a specific one. Useful for provisioning with Puppet when a
node name is used to select the right manifest.

Ensure parent directories are created for base boxes, fixes #1059

Allows setting of validation_client_name from packer template json.

Thanks, @mitchell!

builder/openstack: Add support for standard OpenStack environment variables

post-processor/vsphere: Add disk mode configuration to allow users to specify the desired diskMo...