kubeadm uses certificate rotation to replace the initial high-power
cert provided in --kubeconfig with a less powerful certificate on
the masters. This requires that we pass the contents of the client
config certData and keyData down into the cert store to populate
the initial client.
Add better comments to describe why the flow is required. Add a test
that verifies initial cert contents are written to disk. Change
the cert manager to not use MustRegister for prometheus so that
it can be tested.
Ensure that bootstrap+clientcert-rotation in the Kubelet can:
1. happen in the background so that static pods aren't blocked by bootstrap
2. collapse down to a single call path for requesting a CSR
3. reorganize the code to allow future flexibility in retrieving bootstrap creds
Fetching the first certificate and later certificates when the kubelet
is using client rotation and bootstrapping should share the same code
path. We also want to start the Kubelet static pod loop before
bootstrapping completes. Finally, we want to take an incremental step
towards improving how the bootstrap credentials are loaded from disk
(potentially allowing for a CLI call to get credentials, or a remote
plugin that better integrates with cloud providers or KSMs).
Reorganize how the kubelet client config is determined. If rotation is
off, simplify the code path. If rotation is on, load the config
from disk, and then pass that into the cert manager. The cert manager
creates a client each time it tries to request a new cert.
Preserve existing behavior where:
1. bootstrap kubeconfig is used if the current kubeconfig is invalid/expired
2. we create the kubeconfig file based on the bootstrap kubeconfig, pointing to
the location that new client certs will be placed
3. the newest client cert is used once it has been loaded
