Enable XSD validation before unmarshalling of xml files into java objects

In existing code here=> https://github.com/apache/incubator-lens/blob/master/lens-client/src/main/java/org/apache/lens/client/LensMetadataClient.java#L71 , there is no schema set for JAXB_UNMARSHALLER instance. This would mean that no validation would happen when JAXB_UNMARSHALLER instance is used to unmarshall xml files into java objects. This jira ticket is to do XSD validation before unmarshalling of xml files into java objects.