'Read and return a cached token, or None if not found.
The token is read from the cached token file, which is basically the
entire RSP response containing the auth element.'

def __getCachedToken(self):

    try:
        f = file(self.__getCachedTokenFilename(), 'r')
        data = f.read()
        f.close()
        rsp = XMLNode.parseXML(data)
        return rsp.auth[0].token[0].elementText
    except IOError:
        return None
