Private Repository Support

Added:

  - Ability to specify both Authenticated and Unauthenticated private Docker repository
  - fix if container doesn't have HTTPS support in apt
  - Added custom image tags if user doesn't want to use git hashes for tagging
  - Ability to *not* overrride current `latest` tag in the repo if so desired
  - Test cases for new features
Does it make sense to just define two parameters registry_login_uri and image instead of building them out of parts. That way we could have a configuration like:
image: docker.mycompany.com/myimage
registry_login_uri: https://docker.mycompany.com/v1/
registry_login: true

It seems like that would both solve all the use cases and also be easier to configure / understand.