Introduce base interface for core resource extensions

Introduce base interface for core resource extensions

The interface can be found in neutron.core_extensions.base.

Adopted the interface in qos core resource extension.

Alos moved qos_extension under neutron.core_extensions.qos. Partially,
this is to avoid confusion around the fact that the module does not
really contain a neutron API extension but core resource extension.

Change-Id: I6f6976aa49694f7ef17afa4e93bc769cd0069f65
Partially-Implements: blueprint quantum-qos-api