mds: change the type of data_pools

mds: change the type of data_pools

	Change the type of data_pools from `set` to `vector`

	In following scenario, we found something strange here.

	Here we have two pools as following:

		- data_pool1    id: 20
		- data_pool2    id: 21

	First, we create ceph fs with `data_pool2` and default data_pool is `data_pool2`.
	Then, add the new data pool `data_pool1`.
	Now, the default data_pool will be data_pool1.

	Because set will sort, we can't remain the default data_pool as we wish.
	(we assume that the default data_pool should be the data_pool that we create cephfs)

	It seems vector is more suitable than set.

Signed-off-by: Vicente Cheng <vicente_cheng@bigtera.com>