This implements executing the clean steps in the conductor. The RPC
API version is bumped to allow async clean steps to call back
to the conductor.
Adds node.clean_step to store the current cleaning operation.
The conductor will get a list of clean steps from the node's drivers,
order them by priority, and then have the drivers execute the steps
in order.
Adds a config option to enable cleaning, defaulting to True.
Related-bug: #1174153
Implements blueprint implement-cleaning-states
Change-Id: I96af133c501f86a6e620c4684ee65abad2111f7b
7589d1faaf