App Role guide update.

Documents issuing both client and server certificates. WIF relies on
client certificates.

Server example documents an example of internal Web service using an
autorenewing internal certificate.

systemd/vault-cert-renew@.timer

@@ -0,0 +1,10 @@
+[Unit]
+Description=Check the %i certificate expiry four times a day
+
+[Timer]
+OnCalendar=*-*-* 00,06,12,18:00:00
+RandomizedDelaySec=30min
+Persistent=true
+
+[Install]
+WantedBy=timers.target