Install yum-plugin-priorities

Cherry-picked from https://review.openstack.org/#/c/93924/

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.

Change-Id: Ibef775c1229107fad7bb542f129ef2a6b8597d2c
Closes-Bug: #1316868
Merge "Install yum-plugin-priorities" into stable/icehouse