ScaleIO Volume Plugin - volume attribute updates

This commit introduces the following updates and fixes:
- Enable scaleIO volume multip-mapping based on accessMode
- No longer uses "default" as default values for storagepool & protection domain
- validates capacity when capacity is zero
- Better naming for PV and volume
- make mount ro when accessModes contains ROM

pkg/volume/scaleio/sio_util_test.go

@@ -115,10 +115,10 @@ func TestUtilApplyConfigDefaults(t *testing.T) {
 	if data[confKey.system] != "sio" {
 		t.Error("Unexpected system value")
 	}
-	if data[confKey.protectionDomain] != "default" {
+	if data[confKey.protectionDomain] != "" {
 		t.Error("Unexpected protection domain value")
 	}
-	if data[confKey.storagePool] != "default" {
+	if data[confKey.storagePool] != "" {
 		t.Error("Unexpected storage pool value")
 	}
 	if data[confKey.volumeName] != "sio-vol" {