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.
* Add the /runningpods/ api endpoint
This adds an API endpoint from the kubelet (/runningpods/). It is
an endpoint on kubelet which is considered a "debug" endpoint, so
it might be worth exposing through the options, but by default
it is exposed in most k8s configs AFAICT.
When setting up the http server we return a cancel function to close all
the listeners down.
The issue here is we set the cancel function to nil and thereby cause a
panic when there is an error and the `defer` attempts to call cancel.
This fix just don't set a named return value for the cancel function to
make sure we don't overwrite it with a `return nil, err`.
This ensures that the `defer` can still call `cancel()`.
This cleans up the CLI code significantly.
Also makes some of this re-usable for providers who want to do so.
This also removes the main.go from the top of the tree of the repro,
instead moving it into cmd/virtual-kubelet.
This allows us to better utilize the package namespace (and e.g. mv the
`vkubelet` package to the top of the tree).
