RAPI: remove required parameters for reinstall

Before c744425f instance reinstall
accepted the "os" and "nostartup" optional query parameters. With that
commit it was changed to allow "os" "start" and "osparams" via body
rather than encoded in the URL. Unfortunately that commit introduced a
bug, which required the "os" parameter to be passed for body requests,
and at least one of "os" or "nostartup" for query request.

This fix makes sure all parameters are optional again.

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