parent-key-improvements

There are two current shortcomings of the parent_key implementation in
congress.
* It's unable to be accessed from a subtable. This is needed in the neutron
  refactoring working to handle how neutron structures the response for
  routers.

* It's column name is always named parent_key. It would be helpful if we
  could have the column in the schema actually reflect it's name.
  For example, router_id per say.

Change the datasource framework code so this can be done. I plan to only
make this change for hdict because we don't think we will need vdict in
the future. If it turns out that we need this for vdict then I'll
add support for it.

To handle specifiy the column name on a parent_key on should add:
'parent-col-name': <NAME> to the translator. If this is not present
then the old behavior will still occur of having the column name called
parent_key.