Code review comment for lp://qastaging/~sergiusens/snapcraft/1500902

A couple of style comments inline.

This change makes sense, because now we don't split the plugins into a python and a yaml file. But I think we need to spend some time cleaning and documenting the api of the parent base plugin. Clearly mark things that are internals, constants and things that are to be extended by children. And write nice sphinxed docs.