Add info property to the Cluster object.

* info is dict of dicts: name -> prop_name -> prop_value,
  for example, {'jobtracker': {'Web UI': 'http://...'}}
* vanilla plugin registers both jobtracker and namenode using info

Implements blueprint service-urls-info

Change-Id: I188c6e2344537a2b4bd75ff417d092a5637afd94
Merge "Add info property to the Cluster object."