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
diff --git a/lib/nova_plugins/functions-libvirt b/lib/nova_plugins/functions-libvirt
index 6fb5c38..258e1a4 100644
--- a/lib/nova_plugins/functions-libvirt
+++ b/lib/nova_plugins/functions-libvirt
@@ -37,7 +37,7 @@
     # and HP images used in the gate; rackspace has firewalld but hp
     # cloud doesn't.  RHEL6 doesn't have firewalld either.  So we
     # don't care if it fails.
-    if is_fedora; then
+    if is_fedora && is_package_installed firewalld; then
         sudo service firewalld restart || true
     fi
 }