nova.py: indented and sorted JSON output

This is simply extracting the JSON indenting and sorting from @carsongee's #7444, to try to break it down into a set of smaller and easier to review PRs.

```
$ plugins/inventory/nova.py --list
{
  "ansvc": [
    "10.128.4.57"
  ],
  "mc3-profilesvc": [
    "10.128.4.56"
  ],
  "undefined": [
    "10.128.4.58",
    "10.128.4.55",
    "10.128.4.51",
    "10.128.4.33"
  ]
}
```