'Get a signed certificate for a cn, san.
cn: The common name to use in the certificate
san: The subject alt name to add to the certificate, or None
Returns a path to a pem file containing the key and cert'

def get_cert(self, cn, san):

    self._ensure_loaded()
    path = os.path.sep.join([self.cert_dir, self._get_file_name(cn, san)])
    if (not os.path.exists(path)):
        self._generate_cert(cn, san, path)
    return path
