cinder-volume will convert the disk locally when boot image from volume
with create new volume. whereas the container only have 10G space in
default, cinder-volume will complain that no space if image is large.

This patch mount cinder volume for cinder-volume and cinder-backup all
the time.

Change-Id: I8c68ea49374dc67839699b81e345cfc390a01fe3
Close-Bug: #1692485