- add unit tests for the restore module
- additional dependencies in requirement
- harden the code that calls restore callbacks
- remove WAL-E related code from postgresql.py
- make sure all parameters are taken from the env variables.
- make sure WAL-E code is not involved if the image is not configured for it
- pick up pg_basebackup as an alternative if WAL-E fails.
The WAL-E support will be switched on in subsequent commits.