Support Port Binding Extension in NEC plugin

Fixes bug 1099894
nova/network/quantumv2/api fill the bridge name according to
binding:vif_type attribute passed from Quantum. Otherwise the bridge
is set to None and launching an instance will fail.

blueprint vif-plugging-improvements

Note that binding:capabilities is under discussion and it may be
changed in the future. This commit just adds NEC plugin support
same as OVS and Linux bridge plugin.

Change-Id: Ifd43c3c53615246187621613f72490b62a09b2d6
Merge "Support Port Binding Extension in NEC plugin"