It is not necessary to use 2 "central" bridges, one for "data" and
one for "external" network simulation.
Also using 2 bridge will cause problems when "external_network_bridge"
option will be removed from L3 agent and it will use integration bridge
as it should be done.
Change-Id: I68ee51cbc148b2bfce0cba8de7cf9fe08df54c96