v2-lib: auto add labels from a top level structure

[stavros-k]

Automatically parse a top level labels list if it exists, and add the labels to each targeted container.
Also checks if there are any non-existent containers and raises an error, to avoid any typos.

Questions will look like this:

  - variable: labels
    label: Labels
    group: Labels
    schema:
      type: dict
      attrs:
        - variable: key
          label: Key
          description: Key
          schema:
            type: string
            required: true
        - variable: value
          label: Value
          description: Value
          schema:
            type: string
        - variable: containers
          label: Containers
          description: Containers
          schema:
            type: list
            items:
              - variable: container
                label: Container
                description: Container
                schema:
                  type: string
                  required: true
                  enum:
                    - value: filebrowser