Make quotes optional when verifying agent cert has been signed

The version of puppet included with PE 2.0 does not print quotes
around the certnames in `puppet cert --list`. This prevented the
harness from detecting signed certs on that version. The updated regex
can now handle both PE 2.0 and newer versions.
Merge pull request #420 from branan/"cleaner"-cert-regex

Make quotes optional when verifying agent cert has been signed