Rename the xenial gate jobs and migrate them 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 rename the ubuntu based gate job so that we do not need to rename
them on every new distro migration.

Making failed job n-v till story#2005227
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: I9dd6e178e512c1bba3b6de65532be13d321cd9de

.zuul.yaml

@@ -9,7 +9,9 @@
       - release-notes-jobs-python3
     check:
       jobs:
-        - freezer-api-ubuntu-xenial
+        # making n-v till story#2005227
+        - freezer-api-ubuntu:
+            voting: false
         - freezer-api-centos-7:
             voting: false
         - openstack-tox-pylint
@@ -17,7 +19,8 @@
         - freezer-tempest-basic-sqlalchemy
     gate:
       jobs:
-        - freezer-api-ubuntu-xenial
+        # Commenting as this is n-v job till story#2005227
+        #- freezer-api-ubuntu
         - openstack-tox-pylint
         - freezer-tempest-basic
         - freezer-tempest-basic-sqlalchemy
@@ -26,10 +29,10 @@
         - freezer-api-opensuse-423
 
 - job:
-    name: freezer-api-ubuntu-xenial
+    name: freezer-api-ubuntu
     parent: legacy-dsvm-base
-    run: playbooks/legacy/freezer-api-ubuntu-xenial/run.yaml
-    post-run: playbooks/legacy/freezer-api-ubuntu-xenial/post.yaml
+    run: playbooks/legacy/freezer-api-ubuntu/run.yaml
+    post-run: playbooks/legacy/freezer-api-ubuntu/post.yaml
     timeout: 7800
     required-projects:
       - openstack-infra/devstack-gate