API Nodes states

Expose the power state and provision state of a baremetal node in the
API, each state is represented by a separate entity where clients can
GET/PUT value from/to it. A generic /state subresource is also exposed
allowing clients to GET all the current states of that node and links
to each one of the specific states.

Partially implements blueprint state-machine

Change-Id: I0dcea268718dc3bcb152880654631f9fd61295e7
Merge "API Nodes states"