Move rapi to GetDaemonPort

Currently rapi is the only daemon which accepts a port option, rather
than querying its own port from services, and failing back to the
default if not found. Changing this to conform to what other daemons do.

Also update the ganeti-rapi(8) manpage

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>