legacy multinode jobs are failing with virtualenv error[1] on subnode.
/opt/stack/new/devstack/lib/infra: line 32: virtualenv: command not found
primary node is all good as devstack-gate install the virtualenv but
for subnode no one install it nor devstack neither devstack-gate.
This is happening in py2 version of jobs like nova-live-migration on
stable/train and stable/stein gate. Also grenade multinode py2 jobs.
Till those jobs moved to zuulv3 native which is targetted for this cycle,
let's install virtualenv for them.
[1] https://zuul.opendev.org/t/openstack/build/61558c18919142a3b3973c09989384b1/log/logs/subnode-2/devstacklog.txt#2677
Change-Id: I706ee6dee23decd34b23f99b7e5d88810f939050
e2f9063b28