Makes Python 3 the base python for all tox venvs and removes
py2 jobs from zuul.yaml.
Remaining:
- move the legacy gate jobs from openstack-zuul-jobs to the os-brick
repo and make them py3 jobs
- adjust setup.cfg to require py3 and remove py2 requirements from
test-requirements.txt and doc/requirements.txt
Partial-bug: #1853372
Change-Id: Ib447656aa213914bafd50380b3821090f93776f8