fix(alerts): use mount point filters for disk space alerts

[rikukissa]

This is an improvement / fix to what we previously had as device names can really be anything. Previously we assumed a disk device name /dev/vdabut in many of our own VMs the device mounted to fs root is actually /dev/vda1.

Alerts are now connected to /hostfs and /hostfs/data

[makelicious]

Let's check the min/max difference for alert. Unifications are nice to have

[makelicious]

We might want to use different aggregator

customMetric": {
                        "aggregation": "min", <-- max here
                        "field": "system.filesystem.used.pct", <-- since this changed
                        "id": "alert-custom-metric",
                        "type": "custom"
                    },

[makelicious]

We could unify the thresholds between the changed actions if they are arbitrary

// "Available disk space in root file system"
    "params": {
      "alertOnNoData": true, // <-- Could be unfied
      "criteria": [
        {
          "comparator": ">", // <-- Could be unified
          "customMetric": {
            "aggregation": "max",
            "field": "system.filesystem.used.pct",
            "id": "alert-custom-metric",
            "label": "",
            "type": "custom"
          },
          "metric": "custom",
          "threshold": [0.7], <-- Could be unified
          "timeSize": 1,
          "timeUnit": "h",
          "warningComparator": ">", // <-- Could be unified
          "warningThreshold": [0.5] // <-- Could be unified
        }
      ]
},

// "Low on available disk space in data partition"
    "params": {
      "criteria": [
        {
          "comparator": ">=", // <-- Could be unified
          "customMetric": {
            "aggregation": "min", <-- max
            "field": "system.filesystem.used.pct",
            "id": "alert-custom-metric",
            "type": "custom"
          },
          "metric": "custom",
          "threshold": [0.8],  // <-- Could be unified
          "timeSize": 1,
          "timeUnit": "h",
          "warningComparator": ">=", // <-- Could be unified
          "warningThreshold": [0.5] // <-- Could be unified
        }
      ]
    },