[Fedora] Devstack fails to install in firewalld less system

The official F20 cloud image does not contains the firewalld,
there is nothing to restart, and it fails the installation.

The previous workaround change, assumed all is_fedoras has
a restartable firewalld without ensuring is it installed at all.

RHEL6 even does not have a firewalld.

Closes-bug: #1329102
Change-Id: I3d119ce48af81b30bf02b01c2cd386612ac6ef90
Merge "[Fedora] Devstack fails to install in firewalld less system"