remove factory and store provider on providers list

[flavioribeiro]

I've discussed with @fsouza about that, maybe storing the provider directly on providers list will reduce the overall complexity of the code. We should remove the factory and create a Validate() to be used on init().

[fsouza]

An important side effect of this change is that we would no longer support configuration files, only environment variables, because packages all inits are called before main, and we read the path and parse the config file in main.

We currently use environment variables, and it is indeed an approach that I prefer, so I'm +1 for this change.

It's worth mentioning that we're breaking the Gizmo way of handling configuration, but that's actually a good thing.

[fsouza]

We recently dropped support for file-based configuration in our APIs, so this is no longer a question.