Support EPEL for RHEL-7

This patch makes EPEL repo installation work on all RHEL versions.
EPEL release is not hardcoded anymore.

Change-Id: Ic2a776e563e1e0538806c5d6632e6a4f946be513
Make prescript template RHEL-7 aware

Netns checking make no sense for RHEL-7. We also don't have openstack-selinux
package yet for RHEL-7.

Change-Id: Ie12ca2700d1817b4c6e3425229d3259fff595810
Merge "Support EPEL for RHEL-7"
Merge "Make prescript template RHEL-7 aware"