Image deletion : more consistent behaviour

When using Ceph, the behaviour of "glance image-delete" is not consistent. If the image is qcow2, it will always be deleted, just as expected. If the image is raw, it will only be deleted if it is not currently used by a VM spawned by Nova; otherwise, the deletion will fail because the image is "in use".

Users would expect to always witness the same behaviour, no matter what the format of the image is : it is a "detail" to them, and should not change the way the API behaves.

This might be fixed by "marking" the image for deletion, and letting the scrubber delete it once it is no longer in use.