Test: scale application to zero units

## Test cases
1. Deploy the primary cluster with 3 units , deploy the second cluster, continuously write ON, create test data, and check data.
2. Scale to 0 units, check shutdown all units.
3. Replace hostpath of the second cluster
4. Scale to 1 unit and check blocked status.
5. Down-scale the unit to 0 check shutdown all units.
6. Replace hostpath of the primary cluster
7. Scale to 1 unit, check the test data
8. Scale to 3 unit, check test data

## Implementation
1. Charm blocked when use another cluster's storage.

Start with the eight listed test cases for scaling the primary and second clusters between zero and three units, including hostpath replacement and data checks. Run or add the relevant integration coverage for the PostgreSQL charm; done means all cases pass and the charm is blocked when another cluster's storage is used.

Written by the indexing model from the issue text.