Migrating legacy base jobs nodeset from Xenial to Bionic
Migrating legacy base jobs nodeset from Xenial to Bionic

We have migrated the zuulv3 job to Bionic during Dec/Jan month.
 - http://lists.openstack.org/pipermail/openstack-discuss/2018-December/000837.html
 - https://etherpad.openstack.org/p/devstack-bionic
But that effort does not move all gate job to Bionic as there are
large amount of jobs are still legacy jobs. All the legacy jobs still
use Xenial as nodeset.

As per the decided runtime for Stein, we need to test everything on openstack
CI/CD on Bionic - https://governance.openstack.org/tc/reference/runtimes/stein.html

This commit moves the legacy base jobs nodeset to Bionic. Below are the
jobs which has been changed:
- legacy-base
- legacy-dsvm-base
- legacy-publish-openstack-artifacts
- legacy-dsvm-base-multinode

These are the base jobs for most of the legacy jobs defined or running on
projects gate. Moving them to Bionic will automatically move most of the
projects legacy jobs on bionic.

These jobs are modified with branch variant so that they will use
Bionic node from stein onwards and xenial for all other stable branches
until stable/rocky.

More Details: https://etherpad.openstack.org/p/legacy-job-bionic

Change-Id: I5b99dfc6aecb5944d6fcc3670ae22897714b2006

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