v0.0.12: Improve ergonomics in several small ways

Breaking changes:
- Rename `only_in_overrides` → `enable_in_overrides` in all config files
- Add `pod_type: placeholder` support for placeholder pods (db, etc.)
- Don't require `--override=test` for `test` command
- Don't start up any pods for `--override test up` unless they
  explicitly mention `test` in `only_in_overrides`