Default value of ES_SCHEME should not have quotes

Currently, if this image is run without passing any ES_SCHEME, the value of 'elasticsearch' in the JSON ends up being: `""http://""+window.location.hostname+":9200"`.

The default value of ES_SCHEME becomes `"http"`, which is wrong; it should not include the quotes; should be just `http`.

This pull request removes the quotes and fixes the problem.