append openvpn ca cert to endpoint /5/openvpn/cert

This merge request wraps private key, ca cert and vpn client cert in tags, which corresponds to the inline file support of openvpn (see https://openvpn.net/community-resources/reference-manual-for-openvpn-2-6/, grep for "INLINE FILE SUPPORT"). This way we can easily distinguish ca cert from vpn client cert. This is only done for APIv5, the 3/cert endpoint will return only vpn client private key and vpn client certificate as before. ⚠ ⚠ ⚠ This change requires changes in the parsing on the client side ⚠ ⚠ ⚠

closes #53 (closed)