Changes power_state and adds last_error field

Made these changes:
* power_state -- always represents current power state.
Any power operation sets this back to "actual" when done
(whether successful or not). It is set to ERROR only when
unable to get the power state from a node.

* target_power_state -- represents the requested destination
of a state transition. Cleared when the transition window is
over (whether successful or not).

* last_error -- string field used to store the last error from
any requested asynchronous operation (eg, whether that was to
change power state, or deploy a node, or anything else) that
started but failed to finish. Cleared when any new asynchronous
operation is started.

Closes-Bug: #1237688

Change-Id: I7e079627b87b2cb1606e677e287dd08dcc87263a
Merge "Changes power_state and adds last_error field"