scheduler: when "pd.replicas = 1" the pod always is pending and cannot be scheduled

[xiaojingchen]

when "pd.replicas = 1", scheduler can not scheduler pod.
the scheduler print the log:

E0325 06:13:12.789915       1 mux.go:107] unable to filter nodes: can't find a node from
nodeMap: map[172.16.4.179:[] 172.16.4.182:[] 172.16.4.177:[] 172.16.4.178:[] 172.16.4.181:[] 172.16.4.180:[]]

then check the tidb-scheduler ha logic, found our scheduler can not support the situation when replicas==1

for nodeName, podNames := range nodeMap {
		podsCount := len(podNames)
		if podsCount+1 >= int(replicas+1)/2 {     
			continue
		}
		if min == -1 {
			min = podsCount
		}

		if podsCount > min {
			continue
		}
		if podsCount < min {
			min = podsCount
			minNodeNames = make([]string, 0)
		}
		minNodeNames = append(minNodeNames, nodeName)
}

the expression if podsCount+1 >= int(replicas+1)/2 is alway true when replicas is 1, we need to fix the expression.
@weekface @tennix PTAL