* Make FS space alerts thresholds configurable (#1)
This makes it possible to tweak the thresholds for
the NodeFilesystemSpaceFillingUp alerts. Which
might be necessary in systems like Kubernetes,
where the image garbage collector runs at 85%,
so it's not a problem that the disk reaches that usage %.
Signed-off-by: iuri aranda <iuri@skyscrapers.eu>