yum-validator: Update messaging to handle more cases

This change updates the output for each check to account for settings
which are stored as content overrides and to yield less error-prone
`yum-config-manager`-based instructions if a suitable version is
available.

Also:

    * cleans up/unifies some of the output
    * adds more intelligent quoting
    * fixes the incorrect syntax identified in Bug 1167247
    * eliminates redundant priority setting instructions

Enterprise bug: 1167247
https://bugzilla.redhat.com/show_bug.cgi?id=1167247
Enterprise bug: 1166270
https://bugzilla.redhat.com/show_bug.cgi?id=1166270
Enterprise bug: 1166276
https://bugzilla.redhat.com/show_bug.cgi?id=1166276
Enterprise bug: 1166277
https://bugzilla.redhat.com/show_bug.cgi?id=1166277