yang model: add service provider feature model

yang model: add service provider feature model

As ODL is starting to add more functionalities like port status update
for networking-odl, the negotiation between networking-odl and ODL
openstack service provider to keep compatibility.
networking-odl is supposed to work with multiple versions of ODL,
multiple instances of openstack service providers. So networking-odl
needs to know what neutron extensions/features is supported by
given ODL version/openstack service provider.
This patch defines a yang model for it so that networking-odl can
retrieve from ODL at start up and configure itself.

Change-Id: I90b6bbc3079559209fb444ab79cc88af6a2dc58a
Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>