extern_physcial_network_for_SR-IOV_vxlan

now the vxlan can support ovs vxlan network but donot support sr-iov vxlan network,
if support the sr-iov vxlan network, it's need to support create a network with parm [--provider: physical netwrok] as following:
neutron net-create ext_net -provider:network_type vxlan --provider:physical_network physnet1 --provider:segmentation_id 1000

if support the ovs vxlan network, there is no need parm [--provider: physical netwrok]

so the parm [--provider: physical netwrok] is a optional parm forcreating vxlan network