Make ControllerManager more useful

This changes `ControllerManager` to `Node`.

`Node` is created from a client where the VK lib is responsible for
creating all the things except the client (unless client is nil, then we
use the env client).

This should be a good replacement for node-cli.  It offers a simpler
API.  *It only works with leases enabled* since this seems always
desired, however an option could be added to disable if needed.

The intent of this is to provide a simpler way to get a vk node up and
running while also being extensible. We can slowly add options, but
they should be focussed on a use-case rather than trying to support
every possible scenario... in which case the user can just use the
controllers directly.

website/data/cli.yaml

@@ -38,7 +38,7 @@ flags:
   default: virtual-kubelet
 - name: --os
   arg: string
-  description: The operating system (must be `Linux` or `Windows`)
+  description: The operating system (must be `linux` or `windows`)
   default: Linux
 - name: --pod-sync-workers
   arg: int