post-processor: add vagrantfile_content option

[amezin]

Allow specifying custom Vagrantfile content as string - so it can be generated using templatefile().

This pull request takes a slightly different approach than #112:

• vagrantfile_content isn't processed by old template engine

• thus it's called vagrantfile_content and not vagrantfile_template_content

• vagrantfile_content can't be set together with vagrantfile_template

Closes #104

[hashicorp-cla-app]

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