Simplify the RPC result framework in backend.py

Since now all functions fail via _Fail, the return True, … is redundant
as all normal return paths have it, and thus the True value can be added
in the ganeti-noded handler.

This means that all functions can now forget about the special result
type, and instead return normally, but signal all failures via _Fail().

Only a few functions must be handled specially (the recursive ones).

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