Correctly deal with nodes with no pm_password set

Registering a nova-baremetal node currently requires pm_password to
be set, or the function will raise a KeyError and not register the
node, which is unintended. Check that pm_password is set before we
start fiddling with it.

Change-Id: I39e1d9f5894496de27ee033b9e785b47c98cfcd9
Merge "Correctly deal with nodes with no pm_password set"