PLT-6976: Elasticsearch capitalisation and tests. (#6839)

* Fixes Elasticsearch to have consistent capitalisation everywhere
  across the code and UI (except the config file unfortunately).
* Adds basic unit tests for Elastichsearch.
* Adds a Elasticsearch docker image to the Makefile to enable testing
  the Elasticsearch feature.

cmd/platform/server.go

@@ -120,8 +120,8 @@ func runServer(configFileLocation string) {
 		einterfaces.GetMetricsInterface().StartServer()
 	}
 
-	if einterfaces.GetElasticSearchInterface() != nil {
-		if err := einterfaces.GetElasticSearchInterface().Start(); err != nil {
+	if einterfaces.GetElasticsearchInterface() != nil {
+		if err := einterfaces.GetElasticsearchInterface().Start(); err != nil {
 			l4g.Error(err.Error())
 		}
 	}