ganeti-cleaner: does 'echo 0' instead of 'exit 0'

This is from Debian Bug#568105.

ganeti-cleaner in one case erroneously calls "echo 0" instead of "exit
0". As ganeti-cleaner is called in the daily cronjob, this leads to a
daily mail containing just "0" with an unconfigured default install.

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