This includes updates to CI config, vendor files, etc.
I've hard-coded the k8s depedency at 1.13.4 to keep it inline with what
we currently have and to make sure a another run of `go mod tidy`
doesn't accidentally update it to an unexpected version.
Thanks to hectorj2f for carrying this along.
* 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.
