Add rhel7 as supported distro

rhel7 testing has been fairly stable with redhatci bot testing beta
and RC builds for some time.  Centos7 testing is now coming online
with the nightly builds.  Thus no need to require the unsupported flag
any more.

Change-Id: Iceab5c1d8b713f1f96b5751cf988fe060cfeff58
Merge "Add rhel7 as supported distro"
Merge commit 'refs/changes/83/86083/2' of git://gold.eng.hq.xensource.com/git/github/openstack-dev/devstack into tmpbranch