add python 3.7 unit test job

See ML discussion here [1] for context.

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

Story: #2004073
Co-authored-by: Corey Bryant <corey.bryant@canonical.com>
Signed-off-by: Sahid Orentino Ferdjaoui <sahid.ferdjaoui@canonical.com>
Change-Id: If7fc1428d98a96d73d90ca261c12c67836329341

.zuul.yaml

@@ -7,6 +7,7 @@
       - openstack-python-jobs
       - openstack-python35-jobs
       - openstack-python36-jobs
+      - openstack-python37-jobs
       - periodic-stable-jobs
       - publish-openstack-docs-pti
       - release-notes-jobs-python3

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 2.0
-envlist = py35,py27,pep8,docs
+envlist = py35,py37,py27,pep8,docs
 
 [testenv]
 install_command = pip install {opts} {packages}