Improve ganeti example cron file

The cron file in ganeti's example directory is now static, and executes
ganeti-watcher in /usr/local/sbin no matter where it's really installed. With
this patch we generate it at build time substituting the right value of
@SBINDIR@ from ganeti.cron.in. We also make sure ganeti-watcher exists and is
executable before running it.

This is targeted at 1.2 as well.

Reviewed-by: iustinp