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 adds a new interface that a provider can implement which enables
async notifications of pod status changes rather than the existing loop
which goes through every pod in k8s and checks the status in the
provider.
In practice this should be significantly more efficient since we are not
constantly listing all pods and then looking up the status in the
provider.
For providers that do not support this interface, the old method is
still used to sync state from the provider.
This commit does not update any of the providers to support this
interface.
* Refactor provider init
This moves provider init out of vkubelet setup, instead preferring to
initialize vkubelet with a provider.
* Split API server configuration from setup.
This makes sure that configuration (which is done primarily through env
vars) is separate from actually standing up the servers.
This also makes sure to abort daemon initialization if the API servers
are not able to start.
