Add SSH_PASSWORD environment variable for wal-g

[barzog]

It seems that during /run/etc/wal-e.d/env/ construction SSH_PASSWORD variable is not set (only SSH_PRIVATE_KEY_PATH), so only public-key auth in ssh is supported for wal-g. But according to this: https://github.com/wal-g/wal-g/issues/1276 public-key auth is not supported yet (and I tested it, problem is still here). So in current state ssh backups is not working at all.