Clear the cache of all of the fileserver backends that support the
        clear_cache function or the named backend(s) only.

def clear_cache(self, back=None):
        '''
        Clear the cache of all of the fileserver backends that support the
        clear_cache function or the named backend(s) only.
        '''
        back = self.backends(back)
        cleared = []
        errors = []
        for fsb in back:
            fstr = '{0}.clear_cache'.format(fsb)
            if fstr in self.servers:
                log.debug('Clearing %s fileserver cache', fsb)
                failed = self.servers[fstr]()
                if failed:
                    errors.extend(failed)
                else:
                    cleared.append(
                        'The {0} fileserver cache was successfully cleared'
                        .format(fsb)
                    )
        return cleared, errors