[post-processor/compress] Add support for compress.

This commit adds support for a post-processor that produces a compressed
archive that can be uploaded to an OpenStack cluster through either
the Horizon web interface or Glance.
Fix user variables to properly evaluate.
Fix warnings about not utilizing test module.

Make these tests green adn them figure out how to test the actual post-processor.
Merge pull request #1081 from johnbellone/compress-post-processor

post-processor/compress Add support for artifact compression.