The postfix configuration was a bit too permissive with `mynetworks`.
This commit:
- adds `reject` at the end of `*restrictions` list
- adds tests for this feature
- It's now possible to choose the logging type - either 'plain' or
'json'
- The code is ready to support multiple integration tests (with
different configurations)
- `OPENDKIM_` and `POSTFIX_` variables are handled properly and recorded
in the corresponding files. (This had a downfall that `bash` now needs
to be installed, so we can probably simplify some of the shell
scripts.)
