Make tested vm images reusable

[arungeorgesaji]

So instead of downloading a new image when asked for the same tested vm,it should use the image downloaded before and then use it,this means that there should be a prompt which asks if new one needs to be downloaded and if not then old one can be used.There should be a configuration file somewhere which maps the latest downloaded image for a certain distro/os and the os name,which should be used to map when the next need arrives and then the old image should be duplicated and the duplicate image should be used(note:the latest image can stay the same even when creating the duplicate image as its just a duplicate).

The tested vm is being used when the user runs "./shp -vm create",at that trime it prompts which tested vm to use and thats the tested vm we are talking about.