[Verify] Fixing issue with discovering images

When we request a list of images to choose one of them for tests,
we should make sure all images are active, they have names and they
are PUBLIC. If images are not public, we will have failures of Tempest
tests as described in the bug.

Closes-Bug: #1564431

Change-Id: Ia2cc1586956dcf1fff4c7859faf39fdd98ba35e2