ceph plugin installation: umount correct device if it existed.

The loopback device is created for ceph osd. If the directory
${storage_data_dir} has been mounted when create disk, we should
umount ${storage_data_dir} instead of ${storage_data_dir}/drives/sdb1.

Change-Id: Ie9fe81c820c485dab9f049cf5a81c02424925728
Closes-Bug: #1689089
1 file changed