Refactor /node/<uuid>/state

* Ironic now has a periodic task that checks the current power state of
the nodes[1] and keep the db updated, so the API does not need to call
rpcapi.get_node_power_state,  it can just rely on the current status in
the DB.

* The API will now return all the states in one request, whereas before
two requests were needed (one for power, and one for provision) to get
all the state information.

* Rename /node/<uuid>/state to /node/<uuid>/states (plural) since it
returns information about both states (power and provision)

[1] https://review.openstack.org/#/c/56530/

Change-Id: I9623a378f8743d4146a9b389d28d4f8b02aaf0e0
Merge "Refactor /node/<uuid>/state"