Accept shutdown timeout from the user

Using the new --timeout option:

- gnt-instance shutdown is changed to accept a timeout
- the opcode is changed to hold one
- the LU is changed to optionally get one
- the rpc is changed to carry one
- the backend is changed to take it as a parameter rather than
  hardcoding it in the function

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