Update design document for global hooks
Update design document for global hooks

Make the design document consistent with the current global post hooks
implementation.

Signed-off-by: Oleg Ponomarev <c100474c6090841e9d55cde74d38d81efd269b8d@google.com>
Reviewed-by: Hrvoje Ribicic <be5828fca31f89218048bec4189e30a581aa65da@google.com>

andir/ganeti,mbakke/ganeti,ganeti/ganeti,ganeti/ganeti,leshchevds/ganeti,leshchevds/ganeti,ganeti/ganeti,leshchevds/ganeti,andir/ganeti,mbakke/ganeti,onponomarev/ganeti,onponomarev/ganeti,andir/ganeti,mbakke/ganeti