<oo-admin-check-sources.py> Bug 1023359, detect disabled RHEL channel and language tweaks

Updated required_repos routine to include RHEL base channel in
results, fixing 1023359.

Replaced both vague and overly-specific RHSM and RHN subscription
advice with link to KB article. The article has instructions for
attaching RHSM or RHN subscriptions for broker and node
systems. Instructions for OSE 2.0 will be included when they are
available.

Made available package detection more efficient by switching to
YumBase.searchGenerator()