Add borgmatic_unknown_unencrypted_repo_access_is_ok option

[nikitka]

This pull request is adding the possibility to push to an unencrypted repository.

[m3nu]

Sure you want this as option? If you only need to do this once, it's safer to do it manually once. Similar to the borg init step.

[nikitka]

Frankly, I need a way to make it automatic. :) But I've just realised that a encryption is the better way.