Move ubuntu-bionic into gate
Move ubuntu-bionic into gate

With the release of ubuntu-bionic, we can switch the job to voting and
gate on it.

Change-Id: I0ad08f45d5b0030a24946f1caef52080ac8c914e
Signed-off-by: Paul Belanger <e7139c8e6c4ea75cc1c287ce081b2dc0f3ab39ec@redhat.com>

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