Install yum-plugin-priorities

The yum repository configuration files added by the RDO release make use
of yum priorities. Evaluation of these requires yum-plugin-priorities to
be installed.

Closes-Bug: #1316868
backport: icehouse
Change-Id: I58e69f448d14c1e9ef75bdce97b753e2dfbbe07f
Merge "Install yum-plugin-priorities"