<oo-admin-yum-validator> Bug 1028892, Add warning for --report-all when yum-plugin-priorities missing
https://bugzilla.redhat.com/show_bug.cgi?id=1028892

`--report-all` cannot provide accurate repo priority information when
`yum-plugin-priorities` isn't installed. This commit adds a
high-visibility warning to this effect.
Merge pull request #182 from dobbymoodge/BZ1028892

<oo-admin-yum-validator> Bug 1028892, Add warning for --report-all when yum-plugin-priorities missing