CLIMATE-440 - Remove local PATH_LEADER settings in directory_helpers

- Remove all local copies of PATH_LEADER in test_directory_helpers. Now
  the settings in the backend config are used instead.
- Move PATH_LEADER creation to the test package wide setup function.
  Remove the explicit removes present in test_directory_helpers.
- Remove hardcoded WORK_DIR value in package wide setup function. Now
  the value set in config is used instead.
Resolve CLIMATE-440. Merge PR #38.