Server error with concurrent requests

If there are multiple concurrent identical requests one can trigger a server error due to a race condition:

```
[...] in create_postgres_db cur.execute(create_role) 
IntegrityError: duplicate key value violates unique constraint "pg_authid_rolname_index" 
DETAIL: Key (rolname)=(test_postgraas_delete_user) already exists.
```
Could be solved by introducing a lock or semaphore.