Code review comment for ~kissiel/checkbox-ng:config-refactor-proposable

So the real shortstat of this branch is now:
 19 files changed, 732 insertions(+), 1263 deletions(-)

> Is it doable to get rid of plainbox/impl/secure/config.py completely too?
I did what I can to remove dependencies from that module in this refactor. But to completely remove it we would need to refactor provider_manager that heavily depends on it.