Hide "Use Default Value" menu from output port

This pull request hides ```Use Default Value``` contextual menu from both input and output ports that do not have default value (i.e. with ```PortModel.DefaultValueEnabled``` set to ```false```).

It is to fix the following defect that is tracked internally:

* [MAGN-643 Use default value should not be available on output port of any node](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-643)