Add running devstack-job to the python-mistralclient repository

A bunch of tests which check client methods are prepared to be run
on mistral devstack job. So, need to add running this job for
python-mistralclient repository.

Change-Id: I9aeddc8ed12bece46156f216f505f1fd96b63c4d
Merge "Add running devstack-job to the python-mistralclient repository"