Fix OFPQueuePropMinRate class
Fix OFPQueuePropMinRate class

- __str__ method: needs to pass a tuple to msg_str_attr
- parser method: rate shouldn't be a tuple.

Signed-off-by: FUJITA Tomonori <93dac1fe9c4b2a3957982200319981492ad4976e@lab.ntt.co.jp>

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