tox: use container latest tag for upgrades
tox: use container latest tag for upgrades

Currently tag-build-master-luminous-ubuntu-16.04 is not used anymore.
Also now, 'latest' points to CentOS so we need to make that switch here
too.

We know have latest tags for each stable release so let's use them and
point tox at them to deploy the right version.

Signed-off-by: Sébastien Han <62d754cc350e84d3b1c32ae79f976f5348e74a40@redhat.com>

fgal/ceph-ansible,ceph/ceph-ansible,font/ceph-ansible,ceph/ceph-ansible,fgal/ceph-ansible,font/ceph-ansible