create a smarter way to detect elements in the base datamodel

[mauriciovasquezbernal]

Elements that are part of the polycube base datamodel have to be marked in order to avoid generating code for them, as handlers, serializers and so on.

Currently those elements are marked "by hand" in the pyang code [1], this is not efficient at all as modifications to the base datamodel require to update pyang as well. Even worse, it is possible that one misses to update those elements like in #7.

I think this is possible to extend pyang to mark those elements automatically.

[1] https://github.com/polycube-network/pyang-swagger/blob/master/pyang/plugins/swagger.py#L395