offload: Decompose 'machine-load' into simpler procedures.

* guix/scripts/offload.scm (machine-load): Remove.
(node-load, normalized-load): New procedures.
(choose-build-machine): Call 'open-ssh-session' and 'make-node' from
here; pass the node to 'node-load'.
(check-machine-status): Use 'node-load' instead of 'machine-load'.  Call
'disconnect!' on SESSION.