'Create and store a FakeScalePolicy.'

def add_policy(self, **kwargs):

    allowed = ['scaling_group', 'name', 'policy_type', 'cooldown', 'change', 'is_percent', 'desired_capacity', 'args']
    self._check_args(kwargs, allowed)
    policy_id = str(next(self.policy_counter))
    policy = FakeScalePolicy(policy_id, **kwargs)
    self.policies[policy_id] = policy
    return policy
