FEATURE - repo improvements

* Adding full repo data to metadata available in kickstart templating
  so that we can begin to move away from the magic $yum... variables.
* The above also allows us to create local repo lines in preseeds,
  which is now a new snippet (preseed_apt_repo_config). A late-command
  version could be used to create a 'clean' sources.list too.
* Added apt-specific fields to repo objects in order to make it
  much easier and saner to sync apt repos. Rather than one repo folder
  per dist (like it was before), multiple dists can now go in one folder
  which is how it is on actual apt mirror sites. Also added the option
  to specify multiple apt components (ie. main, restricted, universe) so
  those will all go into the same folder as the dists.
* Modified signatures import so that the new apt repos are created per
  the new model from above.