Remove devstack-gate-periodic-jobs.yaml

Move the remaining periodic-only jobs to devstack-gate.yaml and put
them in the standard devstack-jobs group.  Invoke the devstack-jobs
group with 'periodic' as the pipeline argument to create the
periodic jobs.

Change-Id: I735ac73274deff92d3ed2706939cf60b5506221b
Switch periodic-swift-dsvm-functional to use the template

Use the main swift-functional template to create the periodic
variant of the job.

Change-Id: Ie522915b0340192a6d4f8577c67e4bc58023ba52
Merge "Remove devstack-gate-periodic-jobs.yaml"
Merge "Switch periodic-swift-dsvm-functional to use the template"