rename zones to serverSets to avoid terminology conflict (#10679)

we are bringing in availability zones, we should avoid
zones as per server expansion concept.

cmd/erasure-common_test.go

@@ -55,8 +55,8 @@ func TestErasureParentDirIsObject(t *testing.T) {
 		t.Fatalf("Unexpected object name returned got %s, expected %s", objInfo.Name, objectName)
 	}
 
-	z := obj.(*erasureZones)
-	xl := z.zones[0].sets[0]
+	z := obj.(*erasureServerSets)
+	xl := z.serverSets[0].sets[0]
 	testCases := []struct {
 		parentIsObject bool
 		objectName     string