Neutron: Define a utility function to add neutron service class

When enabling neutron service (i.e. enable_service q-xxx), related code
unconditionally adds a necessary plugin class to Q_SERVICE_PLUGIN_CLASSES.
Which may cause duplication in Q_SERVICE_PLUGIN_CLASSES when
Q_SERVICE_PLUGIN_CLASSES is explicitly specified in localrc.
As a result, neutron server fails to start.

This patch introduces a utility function to add service class,
and check duplication.

Closes-Bug: #1261291
Change-Id: Id2880c7647babfccc3e8d9fc60dd93c4b3997ed9
diff --git a/lib/neutron_plugins/services/vpn b/lib/neutron_plugins/services/vpn
index b8f5c7d..1ab07cb 100644
--- a/lib/neutron_plugins/services/vpn
+++ b/lib/neutron_plugins/services/vpn
@@ -15,11 +15,7 @@
 }
 
 function neutron_vpn_configure_common() {
-    if [[ $Q_SERVICE_PLUGIN_CLASSES == '' ]]; then
-        Q_SERVICE_PLUGIN_CLASSES=$VPN_PLUGIN
-    else
-        Q_SERVICE_PLUGIN_CLASSES="$Q_SERVICE_PLUGIN_CLASSES,$VPN_PLUGIN"
-    fi
+    _neutron_service_plugin_class_add $VPN_PLUGIN
 }
 
 # Restore xtrace