Regions won't be replicated under some conditions

[overvenus]

Three TiKVs run on the same machine, and share the same HDD. When the HDD is almost fall, PD stop replicating regions, even if there is only one replica of those regions. It seems that storageThresholdFilter filters all stores if they are all almost full. I think we should guarantee enough replicas first then take case about storage threshold.

Cc @siddontang @disksing @nolouch

[siddontang]

This is a corner case, and we don't suggest user run multi TiKVs on one disk. We can consider fixing it later, not now.

[overvenus]

It occurs when all nodes are almost full, not only in the above circumstances.