Adjust NodeFilesystemSpaceFillingUp thresholds according default kubelet GC behavior

Previously[1] we attempted to do the same, but there was a
misunderstanding about the GC behavior and it caused the alert to be
fired even before GC comes into play.

According to[2][3] kubelet GC kicks in only when `imageGCHighThresholdPercent` is hit which is set to 85% by default. However `NodeFilesystemSpaceFillingUp` is set to fire as soon as 80% usage is hit.

This commit changes the `fsSpaceFillingUpWarningThreshold` to 15% so
that we give ample time to GC to reclaim unwanted images. This commit
also changes `fsSpaceFillingUpCriticalThreshold` to 10% which gives more time to admins to react to warning before sending critical alert.

[1] https://github.com/prometheus-operator/kube-prometheus/pull/1357
[2] https://docs.openshift.com/container-platform/4.10/nodes/nodes/nodes-nodes-garbage-collection.html#nodes-nodes-garbage-collection-images_nodes-nodes-configuring
[3] https://kubernetes.io/docs/reference/config-api/kubelet-config.v1beta1/



Signed-off-by: Arunprasad Rajkumar <arajkuma@redhat.com>