Get a Twisted Agent that performs Client SSL authentication for Koji.

def _ssl_agent(self):
        """
        Get a Twisted Agent that performs Client SSL authentication for Koji.
        """
        # Load "cert" into a PrivateCertificate.
        certfile = self.lookup(self.profile, 'cert')
        certfile = os.path.expanduser(certfile)
        with open(certfile) as certfp:
            pemdata = certfp.read()
            client_cert = PrivateCertificate.loadPEM(pemdata)

        trustRoot = None  # Use Twisted's platformTrust().
        # Optionally load "serverca" into a Certificate.
        servercafile = self.lookup(self.profile, 'serverca')
        if servercafile:
            servercafile = os.path.expanduser(servercafile)
            trustRoot = RootCATrustRoot(servercafile)

        policy = ClientCertPolicy(trustRoot=trustRoot, client_cert=client_cert)
        return Agent(reactor, policy)