Do not rely on signed images in ephemeral encryption test

The ephemeral encryption might be enabled but nova barbican integration
is not enabled.
The patch allows to use CONF.compute.image_ref in such cases.

Related-Prod: PRODX-14559
Change-Id: I8dfb5d5f32bba4f9260195157539f7b084bc7ac4
(cherry picked from commit 2f4aa738efdd618d78e180a31c1372da100d80c6)
(cherry picked from commit 9dfed2143f386c20fac303a337b9cb97d9301a4d)
1 file changed