don't support rbd_flatten_volume_from_image for ceph-rbd-volume-driver

[zhaoleilc]

Description
===========
The option rbd_flatten_volume_from_snapshot allow users to
flatten volumes created from snapshots to remove dependency
from volume to snapshot so that users can delete snapshot
after they have create volumes from this snapshot.

However, there is a lack of option which allow users to flatten
volumes created from images to remove dependency
from volume to images.

Steps to reproduce
==================
1. Deploy ceph as glance and cinder backend.
2. Upload an image of raw format to glance.(The qcow2 images don't
have this problem)
3. Create a volume from the image.
4. Delete the image.

Expected result
===============
The image can be deleted successfully.

Actual result
=============
The image does not allowed to be deleted.

Environment
===========
The master version of Openstack and the latest
version of ceph.

Logs & Configs
==============
root@devcon:~# openstack image delete cirros_raw
Failed to delete image with name or ID 'cirros_raw': 409 Conflict: Image 641215dd-38ae-4ee4-999c-3d87f886baf7 could not be deleted because it is in use: The image cannot be deleted because it is in use through the backend store outside of Glance. (HTTP 409)
Failed to delete 1 of 1 images.