Hyper-V Agent decomposition

Hyper-V Agent decomposition

Decomposes Hyper-V Neutron Agent and adds the requirements.txt
file to set the dependency to the vendor agent.

Adds README containing information on how to properly use the
Hyper-V Neutron Agent and the ML2 Mechanism Driver.

Vendor plugin code is available on stackforge:
https://github.com/stackforge/networking-hyperv

Plugin already available in pypi:
https://pypi.python.org/pypi/networking-hyperv

DocImpact

Partially-implements: blueprint core-vendor-decomposition

Change-Id: Iedff2718732c884c297cb0be855593057bd64c38