Test: scale application to zero units

## Test cases
1. Deploy primary cluster with 3 units , deploy second cluster, continuous writes ON, create test data, check data.
2. Scale to 0 units, check shutdown all units.
3. Scale to 1 unit with attach storage, check test data.
4. Scale to 2 unit with other cluster's storage, check charm blocked.
5. Scale to 1 unit, check test data.
6. Scale to 2 unit without attached storage, check test data, check storage provided has been reused
7. Scale to 3 unit without attached storage, check test data, check storage provided has been reused

## Implementation
1. Charm blocked when use another cluster's storage.
2. Charm blocked when use different Postgresql versions.