CLOUDSTACK-6505: XenServer bridge for the OVS tunnel network gets reset
on the hosts in the xenserver cluster

this fix ensures that brige is created only once so that openflow rules
configured on the bridge are not lost.
CLOUDSTACK-6507: ensure sequence numbers are honoured while processing
OvsVpcPhysicalTopologyConfigCommand and OvsVpcRoutingPolicyConfigCommand

fix ensures only latest updates are applied (new openflow rules) to the
bidge enabled for distributed routing.