elasticsearch output should add app- prefix to index names.

After talking with Christian I have the final FINAL version of the JSON proposal https://github.com/openshift/enhancements/pull/780

There are small changes, but worthwhile in terms of clarity given the confusion my original proposal caused:

1. Log records stores the index name in elasticsearch.index.
   • Concern was raised that "index" is too generic and may clash.
   • This is consistent with how we store optional data of other kinds.
2. Renamed elasticsearch API fields: structuredTypeName, structuredTypeKey
   • These are not index names, they will get an 'app-' prefix. Calling them "index" caused internal confusion.
3. "app-" prefix added to structured type to form index name.
   • Required to fit into our existing Elasticsearch index model.