Run docs tags only plublish on Xenial
Run docs tags only plublish on Xenial

I don't think we can switch on branch name for these release queue jobs
because the ref info will always be fore refs/tags/*. Just move forward
to running this on Xenial since the vast majority of our future releases
that need publishing will have been tested on Xenial.

Change-Id: I855a2242af4644dd86117812a5efab4643a3f771

Tesora/tesora-project-config,Tesora/tesora-project-config,openstack-infra/project-config,openstack-infra/project-config