[beta8] HMC power driver regression -- Not able to connect via SSH.

From MAAS 1.9 upgrade to MAAS 2.0 we are hitting this stacktrace with known working power parameters:

==> /var/log/maas/rackd.log <==
   File "/usr/lib/python3/dist-packages/twisted/python/threadpool.py", line 262, in <lambda>
     inContext.theWork = lambda: context.call(ctx, func, *args, **kw)
   File "/usr/lib/python3/dist-packages/twisted/python/context.py", line 118, in callWithContext
     return self.currentContext().callWithContext(ctx, func, *args, **kw)
   File "/usr/lib/python3/dist-packages/twisted/python/context.py", line 81, in callWithContext
     return func(*args,**kw)
   File "/usr/lib/python3/dist-packages/provisioningserver/drivers/power/hmc.py", line 97, in power_query
     % (context['lpar'], e))
 provisioningserver.drivers.power.PowerActionError: HMC Power Driver unable to power query lpar alpine01: Could not make SSH connection to HMC for None on None - No authentication methods available