RAPI client: Update ReplaceInstanceDisks

- The RAPI client shouldn't check the replacement mode as only
  the server knows which ones are valid (constants are still
  provided for convenience)
- Disks shouldn't be a list of names, but of indexes

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>