rapi client: accept arbitrary shutdown arguments

The "ignore_offline_nodes" parameter is unsupported. Rather than
explicitely adding it, just pass all keyword arguments in the body of
the query, and rapi on the other side will do the right thing.

Supports for old arguments that were passed via the query is unchanged.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>