Refactor towards better CAPI provider structure and improve code quality

[twelho]

The CAPEI source code is nearly 1-to-1 moved over from https://github.com/weaveworks/wksctl, with little in terms of changes to improve integration/functionality as a standalone CAPI provider/set of controllers. With this separation now done also on the wksctl side (https://github.com/weaveworks/wksctl/pull/286) it should be more straightforward to start restructuring the project for easier maintainability and better code quality.