Add external_network_bridge parameter to the L3 configuration

Because openSUSE uses 'external_network_bridge =' by default in the
l3_agent.ini configuration file we should ensure that external_network_bridge
used the correct value (br-ex).

Change-Id: I3911b01f3618825fe46e20d9a61f8df3022f51b8
Merge "Add external_network_bridge parameter to the L3 configuration"