Add rpc agent api and callbacks to resources_rpc

Add rpc agent api and callbacks to resources_rpc

This patch also refactors existing test cases for server side rpc
classes in order to test code in generic manner. Finally, we remove
notify() and get_resource() from consumers or producers modules
respectively in order to remove circular dependencies. The notificitaion
driver will send events directly using RPC api class instead of going
through registry.

Co-Authored-By: Miguel Angel Ajo <mangelajo@redhat.com>
Partially-Implements: blueprint quantum-qos-api
Change-Id: I9120748505856acc7aa8d15d896697dd8487bb02