The health API reports the affected resources in case of an unhealthy
deployment. Until now all indicators reported one type of resource per
diagnosis (index, ILM policy, snapshot repository)

With the introduction of the disk indicator we now have an indicator
that reports multiple types of resources under the same diagnosis (ie.
nodes and indices).

This changes the structure of the `affected_resources` field to
accommodate multiple types of resources:
```
"affected_resources": {
  "nodes": [
    {
      "id": "e1af6F5rTcmgpExkdOMzCg",
      "name": "hot"
    },
    {
      "id": "u_wBVl4ZRne4uZq_ziLsuw",
      "name": "warm"
    }
  ],
  "indices": [
    ".geoip_databases",
    "test_index"
  ]
}
```