Rename nodes/<uuid>/state to nodes/<uuid>/states

The commit https://review.openstack.org/#/c/60299 refactored the /state
subresource of nodes in the Ironic API and one of the changes made was
to rename the subresource from /state to /states (plural) since now both
power and provision state can be retrieved in one request call.

Change-Id: I3e3cf29fa0fd48063ae0ca14eac6f685ccc44e65