Add location manage mechanism

Now,when user delete an image, the image's file will be deleted as well.This will cause some problems.

1.Create an image 'A' with image file and create another image 'B' refer to the A's location. Then delete image B. The image file will be deleted as well. Finally, A can't be use anymore. But the image file was uploaded by A and A did nothing.User may confuse that why an active image (A) can't be use.

2.Suppose there is an image file in store backend already. Create two images 'A' and 'B' with this image file. Then delete one of the image. The image file will be deleted and the other image could not be use anymore. But the image file doesn't belong to the Glance. If another Glance use the same backend file in image C at the same time. C can't be use any more.

So IMO, we need a mechanism to distinguish the location file's owner.

In the first situation, the image file belongs to A. So when delete B , image file should not be deleted. It will only be deleted when A is deleted.

In the second situation, the image file doesn't belong to any image. So whenever A,B or C be deleted. the image file should not be deleted as well. It could be deleted by store backend administrator.