Repackage discovery registry so that it can be converted to JSON.

The registry uses a non-string type as keys which is not possible in JSON.
Set content type on discovery rest request.
Provide a data-list of locally discovered nodes when adding a new node.
Make cacheEntry public so that it can be marshalled to the UI.
Merge pull request #848 from pluby/discovery

Simpler entry of locally discovered nodes