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: Ib79d3d2bfe52ae0112fd326b0dff07a56ee2bde7
Story: #2004073

openstack/networking-bgpvpn,openstack/networking-bgpvpn,openstack/networking-bgpvpn