post-processor/vagrant/cloudstack: Vagrant CloudStack post-processor

Add support for CloudStack Vagrant post-processor targeting the Vagrant
Cloudstack Provider. The Vagrantfile embedded inside the box contains
the template id and the CloudStack API URL as the built template is only
valid for that particular CloudStack instance.
builder/cloudstack: Expand template name in user data

Allow the use of template name in user data same as other builders.