devstack neutron lib needs to find some functions named
'neutron_plugin_configure_XXXXXX' in the neutron plugin lib
Part of them are usually in the plugin agent file.
For linuxbridge plugin some of theses functions are missing.
This change load the linuxbridge_agent file in linuxbridge file to make
missing functions available to neutron lib when linux bridge plugin is used.
Fix bug #1195237
Change-Id: I567bedc84e77c041e9418773fba1f74b33bcf604