CLOUDSTACK-6925: [OVS] get rid custom logic to create bridges on hosts
in the XenServer pool for GRE tunnel networks

Fix uses XenServer recommended way
Network.other_config:assume_network_is_shared=true
which ensures bridge is created automatically on hosts in the pool for
GRE tunnel networks. Fix also gets rid of  error prone custom logic that ensures
bridge is created by plugging a VIF into the dom0 and connected to
GRE tunnel network.