Use the aliases of plugin in Neutron.conf

The configuration documentation should use the short-hand names
of plugins that is set in setup.cfg instead of forcing admin/operators
to provide a full class path.

Change-Id: Iafb32d79a4615b10ee460d5e38bf9a530d1c1541
Closes-bug: #1332639
Merge "Use the aliases of plugin in Neutron.conf"