Migrate the xenial gate jobs 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

Below patch move the legacy base jobs to bionic which will move the derived jobs
automatically to 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.
- https://review.openstack.org/#/c/639096

This commit migrate the ubuntu based gate job to bionic by removing the
overridden nodeset. parent job nodeset will work here.

More Details: 
- https://etherpad.openstack.org/p/legacy-job-bionic
- http://lists.openstack.org/pipermail/openstack-discuss/2019-March/003614.html
Depends-On: https://review.openstack.org/#/c/639096
Change-Id: I8ed1d446c17d086a491681c5f3da4a3dbaba7ee4

.zuul.yaml

@@ -21,7 +21,6 @@
 - job:
     name: freezer-tempest-basic
     parent: devstack-tempest
-    nodeset: openstack-single-node-xenial
     timeout: 7200
     required-projects:
       - openstack-infra/devstack-gate