This extracts admin kubeconfig generation out of bootkube, now based on
Talos x509 library. On each API request for `kubeconfig`, config is
generated on the fly and sent back on the wire.
This fixes two issues:
* any master node can now generate `kubeconfig` (worker nodes can do
that too, but that should probably change in the future)
* after upgrade-and-wipe the disk scenario, `osctl kubeconfig` still
works
Signed-off-by: Andrey Smirnov <smirnov.andrey@gmail.com>
