Small cleanups:
* Use openstack-lower-constraints-jobs template, remove individual
jobs.
* Sort list of templates
Change-Id: I4c2db4e7e09680fdb64d9b9fc0f2de87976cd190
Needed-By: https://review.openstack.org/623229
We want to default to running all tox environments under python 3, so
set the basepython value in each environment.
We do not want to specify a minor version number, because we do not
want to have to update the file every time we upgrade python.
We do not want to set the override once in testenv, because that
breaks the more specific versions used in default environments like
py35 and py36.
Change-Id: I5ed6fb582e14853e8908c3fb82d5061591ac7bbf