n-cauth serviec has been remvoed from nvoa in Ussuri and that is why
it was removed from default services. But that is requried
for stable/queens|pike|ocata which still depends on text-matrix
for ENABLED_SERVICES. Since stable/rocky we have migrated the
service enable in devstack base job.
This commit fix the Tempest master and stable/queens|pike|ocata
gate by re-enable the n-cauth service for listed stable branches
only. Later we can discuss to remove the test-matrix dependency for
these branches also.