Add EPEL-7 repo for RHEL7

* RHEL7 requires epel for many dependencies.
* The default RDO URL also switched to an icehouse version.
* RHEL6 RDO repo also will be configured for RHEL7, but the
  configuration variable name remains RHEL6_RDO_REPO_RPM for compatibility
  reasons.

Change-Id: I414c0f5357bf24c8d5657ea8231515461862e734
Merge "Add EPEL-7 repo for RHEL7"
Merge branch 'master' of github.com:openstack-dev/devstack