Add legacy-dsvm-base job for converted devstack jobs
Add legacy-dsvm-base job for converted devstack jobs

The dsvm jobs all need an initial set of required-projects. Those
projects are in this job. It was copied from devstack-gate for job
ordering reasons and renamed so that we don't have to do quite as
many steps of the dance.

Change-Id: Iccbfa2a06d4a1f2c39567483467dd2998e9e916b

openstack-infra/openstack-zuul-jobs,openstack-infra/openstack-zuul-jobs