Migrate tests from Xenial to Bionic
Migrate tests from Xenial to Bionic

Switch all Ubuntu Xenial integration test jobs to Bionic (by depending
on the patch that changes them globally.

Change-Id: I5b6c870a12892950c3e8d3aeadffe23ffbf8219e
Depends-On: https://review.openstack.org/639096

openstack/heat,openstack/heat