Migrate rally-database-migration to Ubuntu 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 mofiy the rally-database-migration job to run on
bionic.

More Details:
- https://etherpad.openstack.org/p/legacy-job-bionic
- http://lists.openstack.org/pipermail/openstack-discuss/2019-March/003614.html

Change-Id: Idab37e177430fde3bd6e3a790fe1f15aec06200c

.zuul.d/zuul.yaml

@@ -22,7 +22,7 @@
 - job:
     name: rally-database-migration
     parent: base
-    nodeset: ubuntu-xenial
+    nodeset: ubuntu-bionic
     run: tests/ci/playbooks/rally-database-migration/run.yaml
     timeout: 1800