override-defaults mechanism allows us to define
s_neutron_plugin_security_group before loading lib/neutron,
and we no longer need to have a plugin-specific file in
the master DevStack repository.
Change-Id: I78a4e05fa72b1dd545b8d9e97a8fd8fdbf23739c
Depends-On: Id6765245459c1aff3aab27dbc60d320ce2951b38