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_plugin.go

@@ -149,6 +149,7 @@ var _ volume.PersistentVolumePlugin = &sioPlugin{}
 func (p *sioPlugin) GetAccessModes() []api.PersistentVolumeAccessMode {
 	return []api.PersistentVolumeAccessMode{
 		api.ReadWriteOnce,
+		api.ReadOnlyMany,
 	}
 }