ovn: Run 14 tests with Python 2.7
ovn: Run 14 tests with Python 2.7

The dsvm-functional tox target changed the Python version in upstream
[1] in the Rocky cycle to Python 3 by default. OSP 14 runs on RHEL 7 that
still has Python 2.7 installed. This patch proposes to align with the
default Python version on RHEL 7, which is Python 2.7.

[1] https: //review.opendev.org/#/c/643565/

Change-Id: Ib0151af85d1a6c56e2c296b3ff739512730ee4a1

redhat-openstack/octario,redhat-openstack/octario