ml2: must not plug port until vxlan interface is added

[Thomas Morin]

In some case (e.g. [1]) the neutron agent bagpipe ml2 extension tells bagpipe-bgp to plug a port before the extension has created the bridge and vxlan interface, which results in bagpipe-bgp to fail to reuse the vxlan interface, and instead create a new one. When the interface is finally added, this result in two interfaces being plugged on the bridge, which is a potential source of nightmare.

[1] http://logs.openstack.org/72/562272/1/gate/legacy-tempest-dsvm-networking-bagpipe/d61eb9d/logs/screen-neutron-bagpipe-bgp.txt.gz#_Apr_19_14_52_28_357719