Adds a new constructor to plugin

The new constructor takes an Environment object. This is needed for migration to X-Pack since the environment instance is built by the XPackPlugin and then passed into the feature plugins.

Original commit: elastic/x-pack-elasticsearch@f25225bc6a

elasticsearch/src/main/java/org/elasticsearch/xpack/ml/MlPlugin.java

@@ -141,9 +141,13 @@ public class MlPlugin extends Plugin implements ActionPlugin {
     private boolean enabled;
 
     public MlPlugin(Settings settings) {
+        this(settings, new Environment(settings));
+    }
+
+    public MlPlugin(Settings settings, Environment env) {
         this.enabled = ML_ENABLED.get(settings);
         this.settings = settings;
-        this.env = new Environment(settings);
+        this.env = env;
     }
 
     @Override