pyloxi: reduce boilerplate code required for each module

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.
pyloxi: generate subdirectory from version name
Merge into master from pull request #316:
pyloxi: reduce boilerplate code required for each module (https://github.com/floodlight/loxigen/pull/316)