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
Story: #2004073
Task: #27440

Change-Id: I856881d9a710e01ed171d25c8d2dbf97eb7ed7b3

openstack/oslo.utils