add ca cert to /5/api/openvpn/cert if menshen runs in proxy mode

In case menshen runs in proxy mode in front of an upstream provider, /5/api/openvpn/cert returns the vpn cert and private key, but not the ca cert. Additionally the credentials are not wrapped in the corresponding xml-style tags (<ca>,<cert>,<key>).

Menshen should always return those credentials wrapped in tags in case of APIv5, no matter if it runs in stand-alone or proxy mode.