Add flag category support #796
This adds what I think needs to be done to support categories for flags but we will see if that works. It also forces the scripts to use python2 since they blow up under python3 which is becoming the default python on many linux systems. Small fix to app_test as well so it conforms to the new Flag interface.
Additionally I made some changes to the python generator scripts since python3 is becoming fairly common as the default python. The script currently blows up under python3 though.
I also have some cleanup to do around methods I added that weren't actually needed after digging deeper into the code base.
If you want to use this before it lands since review seems a bit slow on this project right now you can use the following which is an example from my go project. go1.11 required for it.