L2 agent extension manager: read extensions list from config file

L2 agent extension manager: read extensions list from config file

This effectively disables qos extension in the OVS agent, but we don't
rely on it in any functional or fullstack tests so far.

To enable the extension, a user should add:

[agent]
extensions = qos

to their openvswitch_agent.ini file.

DocImpact
Partially-Implements: blueprint quantum-qos-api
Change-Id: Icfbf32c36f98cc6e203841b152c7f6fc4f48c20a