Automatic merge from submit-queue

genericapiserver: extract CA cert from server cert and SNI cert chains

Without this PR a matching server cert or SNI cert is directly used as CA cert in the loopback client config. This fails if the cert is no CA cert.

With this PR the loopback client setup code walks through the chains of the server cert and the SNI certs to find a `CA:TRUE` cert. This is then used as the CA in the loopback client config.