It is very common usecase when neutron-api is intended to run on
infra hosts (inside lxc containers), while neutron-agents are to run on
independent network nodes.
That was not possible by default, so env.d overrides has to be placed
to fix this behaviour. This patch brings option to do this natively
without extra override.
In the meanwhile it shouldn't break any existing depoyments as leaves
previous groups naming as is.
Change-Id: Ie9fa464561c81b503d6946a7afe4870b92790cee
82d7c034a0