Add debian-stable nodeset
Add debian-stable nodeset

Today debian stretch is the current stable release of debian:

  https://wiki.debian.org/DebianReleases

Rather then adding debian-stretch as our default nodeset label, use
debian-stable. This should aid in the transition to debian-buster once
it is released. Since we don't plan to support long term releases of
debian, we can just update this nodeset once we are ready to switch to
buster over updating all projects intree jobs.

Change-Id: I91de606c52bd0571a04da7820214f5ed75c2caa7
Signed-off-by: Paul Belanger <e7139c8e6c4ea75cc1c287ce081b2dc0f3ab39ec@redhat.com>

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