Removed configuration option for qos agent driver selection

Removed configuration option for qos agent driver selection

There is no (general) use case to allow users to configure qos driver to
load by qos l2 agent extension. So instead of getting the driver name
from the configuration file, hardcode it and potentially reuse for other
extensions that may also be interested in splitting extension into
agent agnostic and agent specific pieces.

Added driver_type parameter to AgentCoreResourceExtension.initialize().
Also updated the method signature to reflect that we expect l2
extensions to receive connection.

Finally, removed #noqa import for openvswitch.common.config from qos
extension unit test since it seems unneeded.

Change-Id: Iae4dcc20c967d1da216772a3a3660e0421263527
Partially-Implements: quantum-qos-api