yum-validator: Fixes for RHN and older RHSM

Re-implemented `repofile`-based RHSM repo check for systems with
`subscription-manager` versions which don't support content overrides,
added logic for systems without `yum-plugin-priorities` installed to
prevent useless exception throwing.

Added `CheckSources.override_supported` to indicate that a version of
`subscription-manager` which supports content overrides is installed,
`CheckSources.use_override` now means that `override_supported` is true
and that `subscription-manager` is managing the repository configuration
on this host (i.e. `manage_repos` is set to `1` in
`/etc/rhsm/rhsm.conf`)

Added some missing method documentation

Fixed typos in RHN exclusion advice output, added note for RHN advice to
create `INI` sections that don't already exist, added quotes around
`INI` section names.

Enterprise Bug: 1174193
https://bugzilla.redhat.com/show_bug.cgi?id=1174193
Enterprise Bug: 1175200
https://bugzilla.redhat.com/show_bug.cgi?id=1175200
Enterprise Bug: 1174194
https://bugzilla.redhat.com/show_bug.cgi?id=1174194