Enforce the new providers configuration. Refs #239

Enforce the new providers configuration. Refs #239:

* Updated the `old_to_new()` function to produce a more salt like syntax.
* Added a cli option argument to allow providing a different file which would contain the several cloud providers configuration.
* Besides the separate file for the could providers configuration, we also added support to include any `*.conf` file from `cloud.providers.d/` which is relative to the parent directory of the provided cloud providers configuration file.
* Update the several documentation files in order to include examples of both the old and new cloud providers configuration syntax side-to-side.