Change python3.5 job to python3.7 job on Stein+
Change python3.5 job to python3.7 job on Stein+

python3.5 was the only supported python3 version
on Xenial, now that we have Bionic Beaver nodes
that support python3.7, lets switch to testing
with python3.7 in addition with python3.6 in
Stein and beyond.

See ML discussion here [1] for context.

[1] http://lists.openstack.org/pipermail/openstack-dev/2018-October/135632.html

Change-Id: Ia9868f39ff788e02850fae756820f7c1388bde0a
Depends-On: https://review.openstack.org/#/c/609832/
Story: #2004073
Task: #27427

openstack/manila,openstack/manila