<check-sources> Addressed various review concerns, added some helper functions

This commit mainly fixes concerns raised by Miciah during code review

* Fixed misuse of "Keyword arguments" in docstrings

* Picked a better accessor for getting all configured repos

* Removed side-effects from _yb_no_pri and fixed up dependent
  functions

In addition, some helper functions have been added to
OpenShiftCheckSources:

* repoids([]) returns a list of repoid strings for a list of repos

* enabled_repos, disabled_repos, all_repos and *_repoids convenience
  functions added