OpenVPN: Use a stricter pattern to get the certificate's CN
This addresses Bugzilla issue #10552
The OpenSSL documentation [1] is pretty clear about how the subject is formatted
in a multiline output. Thus, the regex to grep the CN can be made much stricter.

[1] https://www.openssl.org/docs/apps/x509.html#NAME_OPTIONS
OpenVPN: Improve matching of CNs
This addresses Bugzilla issue #10552
This commit contains some improvement in matching CNs while remaining compatible
with possible incorrectly saved CNs in ovpnconfig.