Move MK checkin `hw_id` from URL into JSON body

The MK checkin API had a somewhat inconsistent input: the node `hw_id` field
was supplied inline to the URL, but the facts were supplied in a nested JSON
map, allowing for other inputs.

This pulls down the `hw_id` from the URL into another field in the input JSON,
next to facts, to make the interface uniform.  Now, one place alone needs to
be consulted to understand what is happening -- and the JSON document is a
stand-alone record of what will change.

(That is: the document contains all the data required to reproduce the act, so
capturing the body alone is sufficient to have an audit trail of operations.
It still does not capture the intended operation -- that is "implicit" in the
surrounding environment, so this is not entirely sufficient to provide the
audit facility.)

Signed-off-by: Daniel Pittman <daniel@rimspace.net>