ceph: Allow to change the image path
ceph: Allow to change the image path

If we want to put the disk image into a tmpfs, changing CEPH_DATA_DIR
won't work because some file operation needed by ceph are not allowed
on tmpfs.

So, this change allows to change the disk image path only instead of the
whole CEPH_DATA_DIR.

Change-Id: If92f0bdca16f866a475820cf71a30ded28fb2572

openstack/devstack-plugin-ceph,ajarr/devstack-plugin-ceph