Remove restraint on agent file from ml2 plugin

Ml2 plugin always needs agent file even if the agent is out of tree.
This patch removes the restraint and ofagent_agent.

Change-Id: I12de58e13da1fd162ad8b632d895779ae7560c3c
Closes-Bug: #1477459

Author: Hirofumi Ichihara

lib/neutron_plugins/ml2

@@ -19,7 +19,9 @@ fi
 
 # Default openvswitch L2 agent
 Q_AGENT=${Q_AGENT:-openvswitch}
-source $TOP_DIR/lib/neutron_plugins/${Q_AGENT}_agent
+if [ -f $TOP_DIR/lib/neutron_plugins/${Q_AGENT}_agent ]; then
+    source $TOP_DIR/lib/neutron_plugins/${Q_AGENT}_agent
+fi
 
 # List of MechanismDrivers to load
 Q_ML2_PLUGIN_MECHANISM_DRIVERS=${Q_ML2_PLUGIN_MECHANISM_DRIVERS:-openvswitch,linuxbridge}