interop-testing: Observe --use_tls to disable TLS with OkHttp

interop-testing/src/main/java/io/grpc/testing/integration/TestServiceClient.java

@@ -338,6 +338,8 @@ public class TestServiceClient {
           } catch (Exception e) {
             throw new RuntimeException(e);
           }
+        } else {
+          builder.usePlaintext(true);
         }
         return builder.build();
       }