Change devstack script to correctly configure driver

With the OVN devstack code now in-tree, either Q_AGENT
or NEUTRON_AGENT can be set to "ovn", but the provider
driver plugin was only checking for the latter. This led
to things not being installed or configured depending on
how local.conf was written. Let's support either.

Change-Id: I230cc82559b4cc86d6443bff89881dba55b52503

devstack/plugin.sh

@@ -25,7 +25,9 @@ function _configure_provider_driver {
 }
 
 function is_ovn_enabled {
-    [[ $NEUTRON_AGENT == "ovn" ]] && return 0
+    if [[ $NEUTRON_AGENT == "ovn" || $Q_AGENT == "ovn" ]]; then
+        return 0
+    fi
     return 1
 }