Stop gating on ubuntu-xenial
Stop gating on ubuntu-xenial

There is no issues running ubuntu-bionic, so stop gating on
ubuntu-xenial to save us CI resources.

Change-Id: I4ccae7bc251a7a9c52ae99dd5cd6ba7299806c80
Depends-On: https://review.openstack.org/623074
Signed-off-by: Paul Belanger <e7139c8e6c4ea75cc1c287ce081b2dc0f3ab39ec@redhat.com>

openstack/ansible-role-diskimage-builder