Enforce signature v4 tests all the time, server defaults to only authenticated requests.

All requests must be authenticated to minio server from now on by using keys generated at
``${HOME}/.minio/users.json`` - from ``minio controller`` during its first time run.

Add a new hidden option ``--anonymous`` for running server in unauthenticated mode.

server-api-signature-handler.go

@@ -110,6 +110,7 @@ func (s signatureHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 			writeErrorResponse(w, r, SignatureDoesNotMatch, r.URL.Path)
 			return
 		}
+		s.handler.ServeHTTP(w, r)
 	}
-	s.handler.ServeHTTP(w, r)
+	writeErrorResponse(w, r, AccessDenied, r.URL.Path)
 }