Disable ironic job for stable/icehouse

The non-voting stable/icehouse Ironic jobs are no longer passing
and have not been for some time.  Get them out of the gate until
they are fixed.

Change-Id: Icd194ec847ab98c739032a3979a8c59f6e887647
Related-bug: #1372632
Merge "Disable ironic job for stable/icehouse"