Ubuntu's gntls package generates an Issuer line that looks like this:
        Issuer: C=US,ST=NY,L=Rochester,O=example.com,CN=example.com CA,EMAIL=hostmaster@example.com

While Red Hat's looks like this
Issuer: CN=Red Hat Emerging Technologies

Note the leading whitespace, and the additional fields in the former.

This patch updates the regular expression to:
 * trim leading characters before "Issuer:"
 * trim anything between Issuer: and CN=
 * trim anything after the next ,

I've tested this against the certool output of both RH and Ubuntu
generated certs.
Signed-off-by: NDustin Kirkland <kirkland@canonical.com>
Signed-off-by: NEric Blake <eblake@redhat.com>