Improve examples for how to use networking-sfc Devstack plugin
Improve examples for how to use networking-sfc Devstack plugin

The original example was a bit odd and probably not what users
would typically want to use. This gives two examples with explanation
for where each is appropriate.

Change-Id: Idd256e5d632476713614b7e8d9bfcb40bcf76179

openstack/networking-sfc,openstack/networking-sfc