baremetal power driver takes **kwargs

Some baremetal power drivers may require different information passed to
them. This patch prepares the way by having the base class take only
**kwargs, and having the driver pass both 'instance' and 'node', instead
of just passing 'node'.

blueprint general-bare-metal-provisioning-framework

Change-Id: Iff91024b1d019b0d07f2cbfe991748e618bfcb18
Merge "baremetal power driver takes **kwargs"