fix regex to check CN from server certificate

fix regex to check CN from server certificate

Currently when the script validates the PKI files and
the certificate 'Subject:' field contains RDNs after
the Common Name (CN), these values are also included,
creating a false result that the CN is not correct.

A small change to the sed regex fixes this issue, by
extracting only the value for CN and nothing else. The
regex is replaced with the exact same regex used to
extract the CN value from the client certificate.