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