add python 3.7 unit test job
add python 3.7 unit test job

This is a mechanically generated patch to add a unit test job running
under Python 3.7.

See ML discussion here [1] for context.

[1] http://lists.openstack.org/pipermail/openstack-dev/2018-October/135626.html

Change-Id: I121b0d1b2c217c14c6589455c0155f975f7793a7
Story: #2004073
Task: #27456

stackforge/python-tackerclient,openstack/python-tackerclient,openstack/python-tackerclient,stackforge/python-tackerclient