integrated optional puppet-lint check to ManifestTest

Within ManifestTest puppet-lint validates the rendered result of Vagrant/manifest.pp.twig. If puppet-lint is not installed on the testing machine the according test is skipped and a warning message is displayed.

The rendered manifest is stored in cache/manifest_lint.pp for better debugging purposes. So .gitignore has to be adepted accordingly.

If the complete downloadable archive is also tested somewhen this check should be integrated there too.

Fixes #19