serverPort add from cli (#5821)

in order to change the serverPort from the codegen cli

modules/swagger-codegen/src/main/java/io/swagger/codegen/languages/FlaskConnexionCodegen.java

@@ -141,6 +141,8 @@ public class FlaskConnexionCodegen extends DefaultCodegen implements CodegenConf
                 defaultValue("default_controller"));
         cliOptions.add(new CliOption(SUPPORT_PYTHON2, "support python2").
                 defaultValue("false"));
+        cliOptions.add(new CliOption("serverPort", "TCP port to listen to in app.run").
+                defaultValue("8080"));
     }
 
     @Override