Always use mustache language

Using a null language just defaults to painless which is wrong for
watcher.

Original commit: elastic/x-pack-elasticsearch@48bdc31695

elasticsearch/x-pack/watcher/src/test/java/org/elasticsearch/xpack/watcher/support/WatcherUtilsTests.java

@@ -206,7 +206,7 @@ public class WatcherUtilsTests extends ESTestCase {
             }
             String text = randomAsciiOfLengthBetween(1, 5);
             ScriptService.ScriptType scriptType = randomFrom(ScriptService.ScriptType.values());
-            template = new Script(text, scriptType, randomBoolean() ? null : "mustache", params);
+            template = new Script(text, scriptType, "mustache", params);
             builder.field("template", template);
         }
         builder.endObject();