Fix octavia-v2-dsvm-scenario-ubuntu-xenial

Recent changes to the octavia-tempest-plugin jobs broke the
octavia-v2-dsvm-scenario-ubuntu-xenial job as it tries to use
python3.6 on a xenial nodeset job.
This patch explicitly forces python2 for this job.

Change-Id: I95fa5f2f2fd17140a0ec892265d84542d613c04a
1 file changed