Don't claim Linux Bridge ml2 driver supports bandwidth limit QoS rules

Don't claim Linux Bridge ml2 driver supports bandwidth limit QoS rules

Since we don't want rule_types API to misbehave by claiming support for
QoS for linuxbridge ml2 driver, let's trade API test for its
service correctness.

Note that we cannot enforce the supported rule_types for actual CRUD
operations because then we would need to disable the whole API test
coverage for rules, and we don't want it.

This suggests we should get Linux Bridge support in some way or another.

Change-Id: I86197d02d0474fd9a55a09efcce6a7380c08c5e0
Partially-Implements: blueprint ml2-qos