Add root fingerprint to pki if certificate given

If a root certificate is provided to init an authority the fingerprint
is not currently stored in the default.json file. This patch simply
stores the fingerprint of the supplied certificate.

pki/pki.go

@@ -289,6 +289,10 @@ func (p *PKI) WriteRootCertificate(rootCrt *x509.Certificate, rootKey interface{
 	if err != nil {
 		return err
 	}
+
+	sum := sha256.Sum256(rootCrt.Raw)
+	p.rootFingerprint = strings.ToLower(hex.EncodeToString(sum[:]))
+
 	return nil
 }