balancer-leader should be disabled when upgrade tikv instance trigger evict-leader

[3AceShowHand]

When upgrade a TiDB cluster with multiple TiKV instances, after the first TiKV instance upgrade, region unbalance among all instances, so the balance-leader start to work. This is not necessary, since each TiKV waiting to be upgraded will evict-leader again.

So we should disable it, to make upgrade TiKV instance more stable and more fast.