* Move all but mock provider out of tree
These have all been moved to repos under github.com/virtual-kubelet.
* Introduce a providers.Store
This essentially moves the the old register/ handling into a first class
object that can be controlled from the CLI rather than through build
tags deep in the code.
This actually would have made it a bit easier to build the provider
repos and makes the cmd/ code more re-usable.
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