Refactor test_db.py unit tests

- Use `CLI` with `mox` stubs instead of `MockCLI`
- Parametrize tests to test all combinations and group tests
- Test `--no-salt` arguments
- For `bin/omero db password --no-salt`, test the CLI output using `cap sys`

Couple of open questions:
- should/can we test `bin/omero db password --user-id xx` without `--no-salt`?
- is there a way to test `bin/omero db script --no-salt` ?