Remove firewalld on Fedora 20 by default

firewalld interacts badly with the libvirt on f20, causing slow-downs
so great that it can timeout the gate.

Developers who want to leave it enabled should set FORCE_FIREWALLD=True

Change-Id: I5252a12223a35f7fb7a4ac3c58aa4a3cd1bc4799
Merge "Remove firewalld on Fedora 20 by default"