[swagger] /v5openvpn/cert returns a gateway

1. checkout current main of menshen
2. run it and go to http://localhost:8443/api/swagger//index.html#/Provisioning/get_5_openvpn_cert
3. It will return a Gateway (should be a string? So just the key/cert)

I looked a bit in the code/git history, but it confused my. This blocks kinda. We have bitmask-core!12 (merged) still open and so I can't use latest menshen in my dev env.