Proposal: json as an each check config

[Nastradamus]

Let's think how to turn each check in a way like this:

{
  "free_space_pgdata": {
    "provider": "ssh",
    "check_cmd": "df -h",
    "commands": {
      "raw_output": "df -h",
      "free_percent": "df -T | awk ...",
      "free_bytes": "df -T | bla bla"
    }
  }
}