This makes the concept of a `Provider` wholely implemented in the cli
implementation in cmd/virtual-kubelet.
It allows us to slim down the interfaces used in vkubelet (and
vkubelet/api) to what is actually used there rather than a huge
interface that is only there to serve the CLI's needs.
This ensures that we can catch certain types of errors from providers
and handle accordingly in the core. There is still more to do here to
improve that but this resolves an immediate need to know why a Delete
failed.
vic provider was not updated since I could not figure out where to get
this information.
