Allow a subtranslator to use a field from the parent table as an id

In the previous version, a subtranslator always computed a new id based on the
subtable's content.  This change allows a subtranslator to use a column from
the parent table in the subtable's id column.

For example, the following translator:
  list_translator = {'translation-type': 'LIST',
                     'table-name': 'subtable',
                     'parent-key': 'foreign-key',
                     'val-col': 'val_col',
                     'translator': self.value_trans}
  translator = {'translation-type': 'HDICT', 'table-name': 'testtable',
                'selector-type': 'DICT_SELECTOR',
                'field-translators': ({'fieldname': 'foreign-key',
                                       'translator': self.value_trans},
                                      {'fieldname': 'foo',
                                       'translator': list_translator})}

Would turn the following objects
 {'foreign-key': 100, 'foo': (1, 2)}
 {'foreign-key': 101, 'foo': (3, 4)}

into these tables:

  testtable  subtable
  100        100, 1
  101        100, 2
             101, 3
             101, 4

Change-Id: I5b337c9bf7ba3af3ecf7732215cba38b6f923a95