The change allows loading several service plugins along with core plugin.

The change allows loading several service plugins along with core plugin.

The following functionality changes were made:
1. Multiple plugins are loaded one per type
2. QuantumManager now holds dictionary {plugin_type: plugin_instance}
   Core plugin is stored there as well
3. Extensions are checked against all loaded plugins
4. Service plugins are specified by service_plugins option in quantum.conf file
5. Provide basic interface for service plugins
6. Introduce dummy service plugin as example and PoC
7. Service plugin's REST calls get corresponding plugin's common prefix
8. Add UTs for new extension framework functionality and for QuantumManager

Implements: blueprint quantum-service-framework
Change-Id: I1d00d6f848937410bccd91c852ff0871a86d7bb8