cacti: fix rrd update problem

http://www.cacti.net/downloads/docs/html/scripts.html

if rrd stops updating, then:

1. create new data sources to replace the old ones from graphs, then delete the old rrd files.
2. restart httpd, and run cli/ scripts to poller_output_empty or rebuild_pollor_cache.
3. reduce system usage by adjusting polling interval in cron and settings, from 1min to 5min.
4. disable unused data sources to lower system usage.
5. check cacti log in utilities to make sure everything is green!!