Update RPC protocol version for Ganeti 2.4

As it's quite easy to forget updating the protocol version (and was
actually forgotten for Ganeti 2.3), it is now derived from the
configuration version (which is equivalent with the Ganeti version). The
protocol version for Ganeti 2.4 will be 2040000.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>