openvpn: upgrade to 2.3.6.

The purpose of this patch as below.

 1. upgrade openvpn to 2.3.6 in order to fix CVE-2014-8104

 2. enable systemd

 3. provide new packages named ${PN}-sample to help user create config file
    easily and check whether is openvpn work.

Signed-off-by: Bian Naimeng <biannm@cn.fujitsu.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>