Pin nodeset for unit test job
Pin nodeset for unit test job

devstack unit test job does not set any nodeset
and so does use default nodeset defined in base jobs
in opendev. When opendev switches the default nodeset to
the latest distro version, devstack unit test job can start
failing. Example:

- https://review.opendev.org/q/I01408f2f2959b0788fe712ac268a526502226ee9
- https://review.opendev.org/q/Ib1ea47bc7384e1f579cb08c779a32151fccd6845

To avoid such a situation in future, let's set the working nodeset
for this job also so that when we cut the stable branch we can
run it on the working distro version. 

Change-Id: I302140778fedf08bc8ba72c453837fa7b8f8f9ae

nawawi/openstack,openstack-dev/devstack,openstack-dev/devstack,nawawi/openstack