ganeti-noded: quit on QuitGanetiException

Accoring to the usage documented in the QuitGanetiException docstring,
if we receive such an exception we'll set the global _EXIT_GANETI_NODED
variable to True, and then return either a valid value or an error
message to the user. This will be the last request we serve, though,
because the main loop will be interrupted and the daemon will terminate.

Reviewed-by: iustinp