Check tiflash available pod numbers and the max replica count configured before scaling in

[DanielZhangQD]

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[DanielZhangQD]

If we set replicas according to the TiFlash doc, for example, set TiFlash replica 2, in this case, during the test, we cannot scale in the TiFlash Pods from 2 to 1, because the store for <cluster>-tiflash-1 cannot be deleted, which is the same behavior for TiKV.
In addition, we have added procedure to handle the above case in the scale in doc for TiFlash.