Substantially improve SFC support documentation
Substantially improve SFC support documentation

The documentation regarding how to enable networking-sfc to work
together with networking-odl/ODL-SFC previously left a great deal
of the steps to take up to the user to investigate and figure out.

This improves the documentation so that all the necessary steps
are outlined.

Change-Id: I3f0a11d9c2cae48d6c139bb3f595becd6803187f

openstack/networking-odl,openstack/networking-odl,openstack/networking-odl