remove external_network_bridge option
remove external_network_bridge option

The external_network_bridge option is deprecated/legacy and being
removed from neutron [1]. This patch removes the external_network_bridge
option iniset from networking-ovs-dpdk devstack plugin.

[1] https://review.openstack.org/#/c/567369/

Change-Id: Ibefaaabdd265925f2d495474782a58efd540edca

stackforge/networking-ovs-dpdk,stackforge/networking-ovs-dpdk,openstack/networking-ovs-dpdk,stackforge/networking-ovs-dpdk,openstack/networking-ovs-dpdk