Workaround bug 1263294 for all images we build.

The workaround in tripleo-image-elements for bug 1263294 only takes
effect when an element depends on some of the other got in
'use-ephemeral'. Explicitly drag use-ephemeral in so that we always
have the workaround.

Change-Id: Iacaaba3b71b80292ad8d0d8ad4354f3c8860a07f
Related-Bug: #1263294