remove external_network_bridge option

The external_network_bridge option is deprecated/legacy and being
removed from neutron (see I07474713206c218710544ad98c08caaa37dbf53a).
This patch removes the external_network_bridge option iniset from
devstack scripts.

Change-Id: I4d9641cc9bb83719c9af1edabb89a63c4c2b1d96
2 files changed