ovs-pki: generate x.509 v3 certificate

ovs-pki: generate x.509 v3 certificate

This patch modifies ovs-pki to generate x.509 version 3 certificate.
Compared with the x.509 v1 certificate generated by ovs-pki, version 3
certificate adds subjectAltName field and sets its value the same as
common name (CN). The main reason for this change is to enable
strongSwan IKE daemon to extract certificate identity string from the
subjectAltName field, which makes OVN IPsec implementation easier.

Signed-off-by: Qiuyu Xiao <qiuyu.xiao.qyx@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>