Run devstack-gate stable jobs on all providers

* .../jenkins_job_builder/config/devstack-gate.yaml: The
devstack-gate stable backward-compatibility jobs previously only ran
on devstack-precise-check nodes. Change to run them on
devstack-precise first, and then fall back on devstack-precise-check
as needed.

Change-Id: Ib8c3e262d9614e51490c1b653dacb4057dab2abd
Merge "Run devstack-gate stable jobs on all providers"