Configure project_network_cidr for tempest

This value defaults to something not (necessarily) in our fixed range,
which will cause spurious test behavior. We know the value for this, so
just configure it properly.

Change-Id: I0ee3b71f509377dc7174ce97575e60ee2095f893

lib/tempest

@@ -236,6 +236,8 @@ function configure_tempest {
         fi
     fi
 
+    iniset $TEMPEST_CONFIG network project_network_cidr $FIXED_RANGE
+
     ssh_connect_method=${TEMPEST_SSH_CONNECT_METHOD:-$ssh_connect_method}
 
     # the public network (for floating ip access) is only available