Conver node_leave_cluster rpc to new style result

This patch converts this rpc call to the new style result, and also
changes in the process the meaning of the QuitGanetiException's
arguments and the node daemon rpc call exception handler.

The problem with the exception handler is that we used a two-stage one,
and the inner used to catch all exception (including this one), so in
the logs we always had an exception logged, instead of the normal
'leaving cluster message'. The patch also adds logging of the
exception's arguments, so that we have a trail in the logs about the
shutdown mode.

The exception's arguments were reversed from the normal RPC results
style. While it makes somewhat more sense for this exception, we change
them such that they match the rpc result format.

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