python2 is EOL. OpenStack and Zuul both are python3 only.
Drop the Python-2 CentOS 7 build host testing, including the tripleo
centos-7 train job.
Drop the old openstack-python-jobs template and move to specific
version templates for what we support.
Drop upper-constraints from the py35 environment, as master of
requirements is now using python 3.6-only libraries.
Drop the Xenial tests for similar reasons (rather than making some
sort of forced install); we are keeping Python 3.5 support just for
Zuul compatability at this point so a tox run is enough.