Add better support for references ($ref) to config schemas and action parameter definition JSON schemas

We should add better support for JSON schema references ($ref) inside JSON schemas used for config schemas and action parameter definitions.

Right now the example shown below works, but it's a hacky. host attribute is only used as a reference definition, but because of the way it's defined and the way it currently works, user could also specify it as a top-level host attribute in the actual config which is not desired.

config.schema.yaml

---
  elastic:
    type: "object"
    required: true
    additionalProperties: false
    properties:
      nodes:
        type: "array"
        required: true
        additionalProperties: false
        items:
          "$ref": "#/properties/node"
          required: true

  # top level definition only used by $ref
  node:
    type: "object"
    required: false
    additionalProperties: false
    properties:
        ip_address:
            type: "string"
            required: true
        port:
            type: "integer"
            required: true

examples.yaml (actual config)

---
   elastic:
     nodes:
       -
          ip_address: "127.0.0.1"
          port: 5555
       -
          ip_address: "127.0.0.2"
          port: 5556

One option is to introduce a reserved top-level attribute inside the config schema which would be used to specify schemas which are only used for references.

For example, something like this:

config.schema.yaml

---
  elastic:
    type: "object"
    required: true
    additionalProperties: false
    properties:
      nodes:
        type: "array"
        required: true
        additionalProperties: false
        items:
          "$ref": "#/definitions/node"
          required: true

  # top level definition only used by $ref
  top_level_ref_schemas:
    node:
      type: "object"
      required: false
      additionalProperties: false
      properties:
        ip_address:
            type: "string"
            required: true
        port:
            type: "integer"
            required: true

Inside the code, we would do some mangling so the actual value / object under top_level_ref_schemas attribute would be moved inside "definitions" of the JSON schema so user could then reference it using this notation - "$ref": "#/definitions/node".

I'm of course also open to better name for the attribute than top_level_ref_schemas. It's worth nothing though that this would be a reserved keyword so it needs to be something which is unlikely to come up as the actual actual parameter name / config attribute name.