Add NeutronExternalNetworkBridge parameter

Add NeutronExternalNetworkBridge parameter

This patch adds a new parameter to configure the
neutron external network bridge. This setting
applies to the bridge used in the Neutron l3_agent.ini file
and can by useful if you wish to set external_network_bridge = ''
in that file.

As part of this fix we also update the environment file for
network isolation so that we automatically set the new
NeutronExternalNetworkBridge to an empty string. This fixes
an issue where overcloud floating IPs did not work correctly
when using the external network interface for floating IP
traffic.

Change-Id: I3bfcda8746780ea0851d88ed6db8557e261cef0d