Yang models for OpenStack Neutron SFC APIs.

Yang models for OpenStack Neutron SFC APIs.

Patch has the yang models for OpenStack Neutron Service Function
Chaining API. These models are based on the OpenStack SFC flow
classifier and port grouping APIs. Following URL contains the list
of OpenStack SFC APIs exposed by networking-sfc project:

References:
http://docs.openstack.org/developer/networking-sfc/api.html
https://github.com/openstack/networking-sfc/blob/master/networking_sfc/extensions/sfc.py
https://github.com/openstack/networking-sfc/blob/master/networking_sfc/extensions/flowclassifier.py

Change-Id: I5c1a20d18b34df5850af7ad13529da4a3f1ba767
Signed-off-by: Anil Vishnoi <vishnoianil@gmail.com>