Add back CentOS/RHEL 7 to supported operating systems
Add back CentOS/RHEL 7 to supported operating systems

This change adds back CentOS 7 and RHEL 7 to supported operating
systems, because these operating systems support Ceph Nautilus.
This change will be reverted once stable/nautilus branch is cut off.

Change-Id: Id66bc5e4ad781a0047420c78f36a43de12fa387d

openstack/puppet-ceph,openstack/puppet-ceph