Update nova driver's power_off() parameters

The nova driver's power_off() function received two new parameters
in commit c07ed15415c0ec3c5862f437f440632eff1e94df (nova).  The
in-tree drivers were all updated.  This updates the Ironic driver
and avoids a traceback during power off.

Change-Id: I0cbe7d954f0bcc16f7249304321a08d457eaba52
Closes-bug: #1359943
Merge "Update nova driver's power_off() parameters"