Added missing core_plugins symbolic names
Added missing core_plugins symbolic names

Some plugins miss short symbolic name in setup.cfg, specifically Nuage
and Metaplugin.

Change-Id: I0198ae98654bbdddf8f2ba8dd3b80d0d447d6341
Closes-Bug: 1324487

citrix-openstack-build/neutron-vpnaas,openstack/neutron-vpnaas,dougwig/x-neutron-vpnaas,openstack/neutron-vpnaas