Update vpn cert parsing for APIv5

with menshen!47 (merged) and bitmask-core!30 (merged) I've introduced a breaking change in APIv5 (in order to provide a menshen API post v3, further context can also be found in menshen#53 (closed)). We're now adding the ca cert to the endpoint /5/openvpn/cert. Moreover the vpn cert, the ca cert and the vpn private key are embedded in tags, see menshen!47 (merged) . Therefore the clients need to adapt their parsing of the response. This issue tracks the required change.