data catalog <-> .dvc files: one file in catalog.yml - no existing .dvc

[shaunc]

• part of Feature: Maintain link between kedro data catalog and .dvc files #16

1. Test fixture:
   • use case for different {CONF_ROOT}
   • create catalog.yml in {CONF_ROOT}/base with one entry "example" pointing to data/example.json, and create that file.
2. Code: use DvcRepo.add() to add the file and create .dvc in appropriate place.
3. Test: given fixture, creates {CONF_ROOT}/base/dvc/example.dvc with appropriate content.