Run karbor job on Xenial
Run karbor job on Xenial

Karbor only has master and stable/newton branches so switch its non
voting devstack job to xenial.

Change-Id: Ic9a67fed161d66419fad45b42a6cd25426658973

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