The external_network_bridge option is deprecated/legacy and being
removed from neutron (see depends patch). This patch removes the
external_network_bridge option iniset from our devstack scripts.
Depends-On: I07474713206c218710544ad98c08caaa37dbf53a
Change-Id: I0fdac5d6b7108a7bdd7ef271843b1349979ae8d8