Add EPEL repository to assumptions.

Added EPEL repository to instructions and modified RDO instruction to
use a consistent approach (using the "release" RPM). Also fixed prompt
which erroneously indicated normal user could write to /etc/yum.repos.d.

Fixes bug 1198273

Cherry-picked from https://review.openstack.org/35843

Change-Id: I706b74594212a73aeecd229d2fce7f5244f555b3
Merge "Add EPEL repository to assumptions." into stable/grizzly