965737 - nodes sync does not support distributions

https://bugzilla.redhat.com/show_bug.cgi?id=%20965737

**Problem**: For distributions, the storage_path on content units references a directory.

**Solution**: Tar up the referenced directory and add a flag to the content unit indicating that the published item is a tarball.  The importer (on the child node) detects the flag and expands (and deletes) the downloaded tarball at the location defined in the storage_path.

Pros:
- simple.

Cons: 
- add time to publishing.  rhel 7 distribution ~400 MB takes ~10-13 seconds and has to be done every time we publish repo.
- files contain all of the files in the directory and thus not downloaded in parallel.  this *may* not have any significant impact on performance.