add epel repo if adding rdo repo

Fixes: bug 1282908
Implements: blueprint centos-rhel-for-common

All RDO packages depend on lots of packages from epel. if we have RDO we
have to have epel.

Change-Id: Ie00b22c77e03a4df6835e1df7523509af8976a35
Merge "add epel repo if adding rdo repo"