Fix empty source_repos attribute of distro on import.

During import, the source_repos attribute is properly calculated
but was not being saved, and as a result, was not populated. Without
this, the yum config for new systems won't point back to the yum
repo created when the distro is imported.
Merge pull request #590 from cn28h/fix_source_repos

Fix empty source_repos attribute of distro on import.