Only add optional repo manually when distro != 7.0

When distro version is RHEL 7, do not try to manually add the optional
repo, as it will come from subman.

Simplify the epel repository placement. Use the same template for RHEL
6 and 7, and select the url based on that inside the template.

Change-Id: Ibefe7ece5f318987c05e17506170b2677742de2b