Mechanisms to move extensions and config into service repos

Mechanisms to move extensions and config into service repos

- Extensions will automatically be loaded from service repos in addition
  to neutron proper, but neutron proper will take precedence.
- Config entries for service repos will be read out of neutron-{service}.conf
  first, and then neutron.conf. After Kilo, they will be read only from
  neutron-{service}.conf.
- Service providers for drivers will be collected from all neutron conf files.

This is review 1 of 3.  The second set will be in the server repos, moving
the extensions.  The third will be in neutron, removing the service exts.

Change-Id: I16b5e5b2bb70717166da14faa975fa2ab9129049
Partially-Implements: blueprint services-split