neutron-functional tests started to fail after a change in
devstack [1].
Function setup_devstack_virtualenv now expects that the variable
PYTHON is set.
Call install_python (as stack.sh would) to make sure python is
installed and the PYTHON env variable is set.
[1] https://review.opendev.org/c/openstack/devstack/+/936961
Closes-Bug: #2091093
Change-Id: I4f90b91abe28436b39f8429a091ce4247286352f