Fix hardcoding of openvpn configurations

In https://0xacab.org/leap/menshen/-/blob/main/pkg/genconfig/serialize.go?ref_type=heads#L35 we hard code openvpn configuration properties like cipher, etc.

Ideally we'd like to have those all come from a single source of truth so that a MR like leap/container-platform/lilypad!94 (merged) doesn't also have to come here and change this code as well.