Code review comment for lp:~blake-rouse/maas/fix-1602721

I dug more into the issue and figured out that it was possible that it would affect other areas of MAAS and it all had to do with how __emittable__ was defined on the Bin class. See the code and the comments to understand better why this is needed.

Monkey patching piston3 is not an option as the the whole emitter module in piston3 is one huge function will functions defined in that function. We have thought about just pulling piston3 into MAAS, but now is not a time to do this since this needs to go back into 2.0.