neutron-openvswitch-agent enter in restarting status in multinode
deployments because br-ex is no longer created on compute nodes
unless DVR is enabled.
This change add a conditional to only add bridge_mappings when
the node is in network or in compute when dvr is enabled.
Change-Id: I9d49158127af7fd78ecbc3d64a7509d1e4ae5672
Closes-Bug: #1688571