Enables client authentification for SSL

Client authentification can be set as with the SSL Properties to
require the client certificate to be signed by the server.
Added jUnit test and test certificates

broker/src/main/java/io/moquette/BrokerConstants.java

@@ -41,4 +41,5 @@ public class BrokerConstants {
     public static final int PORT = 1883;
     public static final int WEBSOCKET_PORT = 8080;
     public static final String HOST = "0.0.0.0";
+    public static final String NEED_CLIENT_AUTH = "need_client_auth";
 }