Hack to send FLUSH commands to remote rrdcached as well as local collectd.

Hi.  Here's a small patch to use the RRDCACHED_ADDRESS environment variable set in the webserver environment (eg: either via export in init script or SetEnv in apache conf), to send FLUSH commands to a remote rrdcached, making sure to use paths relative to the rrd data dir as well.

Let me know if you have any questions.

Thanks,
Brian