pyloxi: reduce boilerplate code required for each module

Reviewer: @andi-bigswitch

All the important inheritance hierarchies are grouped together in python 
modules like "of13.action". Previously creating a new module required 5 code 
changes: the `modules` dictionary in lang_python.py, the `roots` dictionary in 
codegen.py, a new `generate_*` function in codegen.py, and imports in the 
init.py and module.py templates.

The new code gets rid of the `targets` dictionary left over from very old 
loxigen. It uses the result of build_ofclasses to determine the set of modules 
for a particular openflow version. The only code change required to create a 
new module is defining its inheritance root in the `roots` dictionary.