Add node to "known" check fields

[smoyer64]

In #58 it's noted that node is used in the example without being defined in the specification. While I agree that the functionality requested in #58 may be important, I'd suggest that node or some other unique identifier is needed.

The specification states:

Since each sub-component may be backed by several nodes with varying health statuses, these keys point to arrays of objects. In case of a single-node sub-component (or if presence of nodes is not relevant), a single-element array SHOULD be used as the value, for consistency.

Since an array SHOULD be used, I feel like this specification should also identify the field within the check that identifies which instance of the component the enclosed component represents. The word node or nodeId make sense for clustered operation but perhaps instance or instanceId would be more generic.