20190102

Add feature to flatten the volume from glance image snapshort

When using ceph in glance and nova, if show_multiple_locations is
enabled on glance side, nova will clone a COW image from glance. The
image will use glance image as parent. This will result nova rbd pool
depends on glance rbd pool. If glance rbd pool is broken, nova is
affected too.

This patch add a new option `rbd_flatten_volume_from_image`. When it is
True, the image cloned from glance image pool will be flattened.

Closes: #1
Closes-Bug: #1802467
Change-Id: If158103150d11480b64467685b3672db017521b0