Run filesystem collections (df) on each filesystem separately to
avoid complete hangups when e.g. network filesystems are blocked.
Two disk usage thresholds: high and critical.
Filesystem thresholds and event transforms.

Features:
- ZProperties for various thresholds
- replace zenoss.cmd.df with CERIT_SC.cmd.linux.df
- finetune multiple df thresholds
- event text transforms for filesystem threshold violations