<check-sources> Bug 1023374 - detect and fix/advise repo version conflicts

https://bugzilla.redhat.com/show_bug.cgi?id=1023374

Added logic to check for enabled repos with versions different from
detected or specified `oo_version`. Also added `disable_repo` method
to `check_sources.py` so mismatched repos can be disabled.

Bonus: Added "No problems were detected!" so humans can tell that
their repos pass muster.