Add differentiating label for alerts that use nodeCriticalSeverity

[leizor]

This affects the following alerts:

• NodeFilesystemAlmostOutOfFiles
• NodeFilesystemSpaceFillingUp
• NodeFilesystemAlmostOutOfSpace
• NodeFilesystemFilesFillingUp

Each alert is actually two alerting rules, one for the warning level, and one for the critical level. However, the critical one can be set to emit as a warning by using $._config.nodeCriticalSeverity. When this is true, both alerting rules will generate alerts with an identical set of labels, which is an error when both alerts fire.

We add an extra label to these alerting rules in order to differentiate between the two levels when $._config.nodeCriticalSeverity is set to warning.