Better reporting of total/free usable capacity of the cluster (#15230)

The current code uses approximation using a ratio. The approximation 
can skew if we have multiple pools with different disk capacities.

Replace the algorithm with a simpler one which counts data 
disks and ignore parity disks.

cmd/erasure-single-drive.go

@@ -184,6 +184,10 @@ func (es *erasureSingle) Shutdown(ctx context.Context) error {
 	return nil
 }
 
+func (es *erasureSingle) SetDriveCounts() []int {
+	return []int{1}
+}
+
 func (es *erasureSingle) BackendInfo() (b madmin.BackendInfo) {
 	b.Type = madmin.Erasure