Register DEFAULT group on agent start

From upstream commit 4d3a2747 fixing bug (#1968606), registration of the
group DEFAULT does not happen automatically anymore.

If not loaded, agent fails with 'oslo_config.cfg.NoSuchOptError:
no such option api_extensions_path in group [DEFAULT]'

networking_aci/plugins/ml2/drivers/mech_aci/agent/entry_point.py

@@ -27,6 +27,7 @@
 
 
 def register_options():
+    common_config.register_common_config_options()
     config.register_agent_state_opts_helper(cfg.CONF)