Adds support for Quantum networking in Hyper-V

Blueprint quantum-plugin-hyper-v

Adds a VIF plugin model to decouple the Nova Hyper-V compute driver
from the actual networking service in use.
Adds a VIF plugin for Quantum and a plugin for Nova networking.

The VIF driver is selected by mapping the value of the "network_api_class"
option to the corresponding VIF driver class name, avoiding this way an
otherwise redundant configuration option.

Change-Id: If35e814e3e5e181e79249c2f9ea86e878771e4c9
Merge "Adds support for Quantum networking in Hyper-V"