Ensure python3.7 installed for openstack-tox-py37
Ensure python3.7 installed for openstack-tox-py37

Set the python_version variable of the ensure-python role to 3.7,
ensuring that the python3.7 interpreter is installed.

Change-Id: I4b2149cf7033f16a37cd185e5f19ab2e7e837cdf
Depends-On: Ia56cba58c87aa48de3e8e3f3d28f6252f7dcd7c3

openstack-infra/openstack-zuul-jobs,openstack-infra/openstack-zuul-jobs