Switch python3 test to bionic and add it to gate
Switch python3 test to bionic and add it to gate

Ubuntu bionic is python3 by default, and OSA
stein will not support Ubuntu xenial, so it
makes sense to switch the testing to use bionic
instead.

The test is also passing, so we should add it
into the gate queue to prevent regressions.

Change-Id: I43b6aac57f664e19f2d77be5a17fd3cb663075b2

openstack/openstack-ansible-plugins,openstack/openstack-ansible-plugins