smoketest: T7738: provide OpenVPN test certificates for migration tests

This adds necessary embedded certificates to validate the migration logic added
in commit 63cc76f ("openvpn: T7738: avoid duplicate certs during 1.3 -> 1.4
migration").

The CA used was generated using EasyRSA as described in our documentation:
https://docs.vyos.io/en/1.3/configuration/interfaces/openvpn.html#generate-x-509-certificate-and-keys

Author: c-po