Re-enable "vlan" mode for openvswitch

With the latest changes to the network barclamp (disabled vlan for the
"nova_fixed" network) the "vlan" mode for the openswitch driver should work
again.

There is however the limitation, that the bridge for the openvswitch network
needs to be on top a different physical device (or bond) than the "admin"
network.  Otherwise nodes might not come up correctly after a reboot.

Currently the quantum UI will display a warning about that.

A working openvswitch vlan mode is a requirement for the cisco nexus switch
support.