lib/neutron: Use NEUTRON_PLUGIN instead of hardcoding ml2

Change-Id: Ia60381694f30978984cdf33e3141dc153d294e17
diff --git a/lib/neutron b/lib/neutron
index c1552e3..3fbcbd9 100644
--- a/lib/neutron
+++ b/lib/neutron
@@ -139,7 +139,7 @@
 
         cp $NEUTRON_DIR/etc/api-paste.ini $NEUTRON_CONF_DIR/api-paste.ini
 
-        iniset $NEUTRON_CONF DEFAULT core_plugin ml2
+        iniset $NEUTRON_CONF DEFAULT core_plugin $NEUTRON_PLUGIN
 
         iniset $NEUTRON_CONF DEFAULT policy_file $policy_file
         iniset $NEUTRON_CONF DEFAULT allow_overlapping_ips True