registry: don't iterate through certs

the golang tls.Conn does a fine job of that.
http://golang.org/src/pkg/crypto/tls/handshake_client.go?#L334

Signed-off-by: Vincent Batts <vbatts@redhat.com>
Merge pull request #8577 from vbatts/vbatts-registry_certs

registry: don't iterate through certs