Similar to the TPM bootstrapping on GCE (indeed, a lot of the code is
modified from there), but we verify the PKI signature against a public
key in a Host CRD object.
In order to verify that the caller is running on the specified node,
we source the expected IP address from the cloud, and require that the
node set up a simple challenge/response server to answer requests.
Because the challenge server runs on a port outside of the nodePort
range, this also makes it harder for pods to impersonate their host
nodes - though we do combine this with TPM and similar functionality
where it is available.