Merge remote-tracking branch 'origin/feature/qos' into merge-branch

Merge remote-tracking branch 'origin/feature/qos' into merge-branch

Also applied the following fixes:

===

1. cleaned up some pylint failures that were not spotted before:

Module neutron.objects.qos.policy: Metaclass class method __new__ should
have 'mcs' as first argument

Module neutron.objects.qos.rule: Lambda may not be necessary

===

2. Revert "Introduce the AFTER_READ callback for ports and networks"

This reverts commit e3dba1424114575581c153e02227282e036ad0a2.

We don't use callbacks to extend resources anymore, instead relying on
ml2 extension drivers. No need for the patch to achieve QoS, and it also
breaks test_delete_subnet_with_callback that was added in master
recently.

===

3. updated requirements.txt and test-requirements.txt based on:

https://review.openstack.org/#/c/204398/

to avoid requirements gate checks failing due to incompatible
requirements comparing to global-requirements.txt

Change-Id: I744ab2d8327a428a5467f2d07d073a5f8c333520