Default to the ML2 plugin in Neutron instead of OVS

In Icehouse, the OVS and LinuxBridge plugins are being deprecated in
favor of the Modular Layer 2 (ML2) plugin. This change modifies
devstack to default to ML2 when Neutron is used for networking.

Fixes bug 1220745

Change-Id: I9a4c84b04727a710219fc11f862a655309ffb99b
diff --git a/lib/neutron b/lib/neutron
index 4a3d1b0..b1f96fc 100644
--- a/lib/neutron
+++ b/lib/neutron
@@ -88,7 +88,7 @@
 export NEUTRON_TEST_CONFIG_FILE=${NEUTRON_TEST_CONFIG_FILE:-"$NEUTRON_CONF_DIR/debug.ini"}
 
 # Default Neutron Plugin
-Q_PLUGIN=${Q_PLUGIN:-openvswitch}
+Q_PLUGIN=${Q_PLUGIN:-ml2}
 # Default Neutron Port
 Q_PORT=${Q_PORT:-9696}
 # Default Neutron Host