Return the update intervals for all of the enabled fileserver backends
        which support variable update intervals.

def update_intervals(self, back=None):
        '''
        Return the update intervals for all of the enabled fileserver backends
        which support variable update intervals.
        '''
        back = self.backends(back)
        ret = {}
        for fsb in back:
            fstr = '{0}.update_intervals'.format(fsb)
            if fstr in self.servers:
                ret[fsb] = self.servers[fstr]()
        return ret