<oo-admin-check-sources.py> Fix redundant priorities advice

Cache priorities fixes in `resolved_repos` so `--report-all` gives
reasonable output. It was repeating the priorities setting advices for
the base RHEL channel and warning about repo priorities >= 99 after it
advised to set the same repos to a lower priority
Merge pull request #136 from dobbymoodge/check-sources-bugs

<oo-admin-check-sources.py> Fix redundant priorities advice