BZ #1045137 - Ensure physical ports added to ovs bridge on networker.

https://bugzilla.redhat.com/show_bug.cgi?id=1045137

Move configuration of bridge and ports to the bridge_uplinks parameter
of the ::neutron::agents::ovs declaration in networker.pp.

Example value for the ovs_bride_uplinks parameter in networker.pp:
["br-em2:em2","br-p3p1:p3p1"]
(cherry picked from commit 81899980d02040446bc958a2896f9c5f67fc80f1)
BZ #1045132 - l3 external_network_bridge now configurable.

https://bugzilla.redhat.com/show_bug.cgi?id=1045132

$external_network_bridge is used to configure the external_network_bridge
in /etc/neutron/l3_agent.ini .  Set to '' to use a provider
network instead.
(cherry picked from commit 7a7fe553e025dfe2c70b8e283657fb1d63cfdab1)