Drops support for creating Python 2 virtualenvs in bootstrap-servers,
and looking for a python2 interpreter in the kolla-ansible script.
Also forces the use of Python 3 as the remote interpreter in CI on
Debian and Ubuntu hosts, since they typically symlink the unversioned
interpreter to python2.7.
Change-Id: Id0e977de381e7faafed738674a140ba36184727e
Partially-Implements: blueprint drop-py2-support