packer: validate minimum version [GH-2310]

Fix flags so we can do e.g. -only=build1,build2

AppVeyor: only build master branch

AppVeyor builds all branches per default.
Add a configuration setting to only build the master branch.

See http://www.appveyor.com/docs/branches#white-and-blacklisting for details.

For the record, AppVeyor builds can skipped by annotating the commit message
according to http://www.appveyor.com/docs/how-to/skip-build
Useful for e.g. website commits.

[skip appveyor]

Add AppVeyor status badge to README

Add AppVeyor configuration file

Add knife config by template

website: Update docs for default configuration template

builder/vmware: Run stat on vmware when checksum type = none

Fix setting ami_groups

Fixes spot instance cleanup.

website: Update some docs to point to Atlas instead of Vagrant Cloud

website: Tips for debugging a build.

#2346 updated to AMI that doesn't require accepting marketplace terms

builder/aws: fix adding launch permissions for created ami.

Packer lacks an official Windows CI server.
This commit adds a configuration file for AppVeyor, which is a Windows CI
service (free for open-source projects).

Updated to same ami as in main readme.md which is from community AMIs
and therefore doesn't require accepting the AWS marketplace terms
before using in Packer build.

Change docs usage of aws.ami to amazon.ami to be consistent with terraform

Previously, groups were granted launch permissions by submitting a
ModifyImageAttribute request with the UserGroups parameter set
appropriately. This is no longer valid, as the LaunchPermission
parameter must be set.

builder/amazon-instance: Don't specify empty Virtualization Type

Update docs for default configuration template for the chef-client provisioner, to reflect the ability to set a custom validation_client_name