Restore the openvpn.service file from packaging of openvpn 2.3.6, so we
can still start it the same way (as currently described in manuals).
Related: SYS#6303
Change-Id: I2cf96bc1ecca79ae98be1181fbd3c15edad15ca8
Our openvpn server has meanwhile been migrated to 2.5.x, and
establishing backwards compatibility with 2.3.x means we have to
disable ciphers + tls versions that are no longer considered secure.
Related: SYS#6303