zuul/layout: fix rule for beaker-xenial jobs
zuul/layout: fix rule for beaker-xenial jobs

Puppet beakers jobs on Xenial should not be run on stable/mitaka, we
probably missed it when we added the job in the gate.
This patch fix it.

Also ignore xenial-nv jobs on stable branches.

Change-Id: I14ce562f1a870b75633eb7f052ddace484e00e4f

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