Pulp

Add Distribution ViewSet and Serializer

Additional work was done to allow arbitrarily deeply nested ViewSets.

The router registration code in urls.py was significantly changed. A ViewSetNode tree is assembled that mirrors the nesting hierarchy of the url structure, and is able to create child routers as necessary and use them to register child ViewSets.

A parent_viewset attribute is added to ViewSets, which indicates how they are nested and assists in the retrieval of parents.

A new type of field, WritableNestedUrlRelatedField was added to allow parent fields to be writable when the parent is determined by the nested url rather than by request parameters (and designated read_only). Serializers with writable url fields inherit from NestedModelSerializer.