py26 job fail on stable/kilo due to new mock version

[Roman Podoliaka]

We don't specify the dependency on mock explicitly in test-requirements.txt, so it gets installed as a transitive dependency. Which basically means we have no control on the allowed mock version. It turned out, there is cap on the max version of mock in stable/kilo global requirements, but we just can't use it in oslo.db until we have mock explicitly mentioned in test-requirements.

[Roman Podoliaka (rpodolyaka)]

https://review.openstack.org/#/c/204000/2

[Davanum Srinivas (DIMS) (dims-v)]

This issue was fixed in the openstack/oslo.db 1.7.3 release.