Make sure service providers can be loaded correctly
Make sure service providers can be loaded correctly

This patch fixes a regression where, if neutron was loaded using
--config-dir, the service_providers option was no longer available.

We bring the logic back (removed by 61121c5f2af), alongside the ability
to load the option auto-magically. This is especially required for DevStack
deployments as of today, because neutron-server is only loaded by passing
--config-file (...)neutron.conf and --config-file (...)ml2_conf.ini

Change-Id: I9bfaed9e19a5506e27795a0b7ad47f4c31fefa40
Closes-bug: #1490990

sasukeh/neutron,yanheven/neutron,jumpojoy/neutron,noironetworks/neutron,mahak/neutron,openstack/neutron,bigswitch/neutron,takeshineshiro/neutron,huntxu/neutron,shahbazn/neutron,wolverineav/neutron,yanheven/neutron,mahak/neutron,swdream/neutron,JianyuWang/neutron,eayunstack/neutron,openstack/neutron,vivekanand1101/neutron,sasukeh/neutron,apporc/neutron,openstack/neutron,silenci/neutron,huntxu/neutron,asgard-lab/neutron,chitr/neutron,skyddv/neutron,takeshineshiro/neutron,skyddv/neutron,mattt416/neutron,igor-toga/local-snat,igor-toga/local-snat,mattt416/neutron,SamYaple/neutron,cloudbase/neutron,MaximNevrov/neutron,silenci/neutron,jumpojoy/neutron,chitr/neutron,eayunstack/neutron,dhanunjaya/neutron,noironetworks/neutron,wolverineav/neutron,vivekanand1101/neutron,klmitch/neutron,glove747/liberty-neutron,MaximNevrov/neutron,bigswitch/neutron,dims/neutron,asgard-lab/neutron,klmitch/neutron,dims/neutron,SamYaple/neutron,swdream/neutron,apporc/neutron,sebrandon1/neutron,cloudbase/neutron,JianyuWang/neutron,sebrandon1/neutron,mahak/neutron,dhanunjaya/neutron,glove747/liberty-neutron,shahbazn/neutron