Log license type on license update (elastic/elasticsearch#4308)

Now when a cluster gets updated with a new license,
the license uid and type will be logged as info. Making
it easier to verify the current license mode of the cluster.

Original commit: elastic/x-pack-elasticsearch@c205cb42ae

elasticsearch/src/main/java/org/elasticsearch/license/LicenseService.java

@@ -424,6 +424,8 @@ public class LicenseService extends AbstractLifecycleComponent implements Cluste
                     // remove operationModeFileWatcher to gc the old license object
                     previousLicense.removeOperationModeFileWatcher();
                 }
+                logger.info("license [{}] mode [{}] - valid", license.uid(),
+                        license.operationMode().name().toLowerCase(Locale.ROOT));
             }
             updateLicenseState(license);
         }