OVS agent QoS extension functional test for bandwidth limit rules

OVS agent QoS extension functional test for bandwidth limit rules

This functional test spawns the OVS agent, with bandwidth limit rules in
a policy attached to ports. Then it asserts that the low level OVS
bandwidth limits are set for each port.

To make this possible we refactor and extract the base OVS agent test
framework into neutron.tests.functional.agent.l2.base.

Partially-Implements: blueprint ml2-qos
Change-Id: Ie5424a257b9ca07afa72a39ae6f1551d6ad351e7