Disable ironic job on stable/havana

The check-tempest-dsvm-virtual-ironic-nv job is running in check for
nova (and some other repos).  I noticed that it ran against a backport
I did for stable/havana.  It seems like this job is really only
relevant for running against stable/icehouse and up.

Change-Id: I4b85d3b2cdf32afeb51d99abd28bec48cb9a779a
Merge "Disable ironic job on stable/havana"