Fix swift loopback creation

- Device name for swift::storage::loopback cannot contain undescore unless we
  want to specify metadata.

Change-Id: I5631e4cd79ab2a0fdaf7ad206c7e3f6606266daf
Fixes: rhbz#1153354
Signed-off-by: Gael Chamoulaud <gchamoul@redhat.com>
(cherry picked from commit 34065abc59ce904eced78deba1f41f7c53a1b6ab)