neutron.api.rpc.callbacks interface rework

neutron.api.rpc.callbacks interface rework

Split rpc.callbacks interface into consumer and producer parts.

Better terms are chosen for two RPC APIs we have:
- pull when a component actively requests a new object state;
- push when a component updates anyone interested about an object
  change.

Also, for callback registration, the following terms are used:
- subscribe when a component is registered in consumer registry;
- provide when a component is registered in provider registry.

Covered the registries with some unit tests.

Lots of existing tests utilize the registries now, and need to be
isolated from other tests that mess with the managers (that are
singletons), so introduced a common qos base test class to mock the
manager with per-test instance of it).

Co-Authored-By: Ihar Hrachyshka <ihrachys@redhat.com>
Partially-Implements: blueprint quantum-qos-api
Change-Id: I130cfbc8b78da6df4405b90ea1ab47899491ba41