Graphs that were using `file_exists()` to check for the presence of RRD
files now use `rrdtool_check_rrd_exists()` instead. This is a fix for
distributed poller configurations that are running `rrdcached` on a
different host.
Use data_update instead of rrd_update/rrd_create and influx_update
Centralize rrd file check so we can check against a remote rrdcached server too
Use rrd_name() to generate rrd file names
