Externalize self reference url and make the port configurable

[dileepbapat]

Make the schema loader self reference url configurable.

http://localhost:8080/_schemas/

https://github.com/project-sunbird/open-saber/blob/78e493c6ab73f9115d36becd6e6b6adc230360de/java/validators/json/jsonschema/src/main/java/io/opensaber/validators/json/jsonschema/JsonValidationServiceImpl.java#L32