Issue 2353: Update All - enable support for Systems and System Groups

This pull request contains a few commits to allow a user to perform an
'Update All Packages' on either a System or System Group.

Katello had this capability for Systems in pulpv1; however, it was lost as part of the migration to pulpv2.  This pull request brings that functionality back. In addition, it adds support for the same on a System Group.

Note: for this to work 'end to end', the following runcible pull request will also be needed:
https://github.com/Katello/runcible/pull/73