InputScenarioUtils load images if glance available

The scenario_images utility of InputScenarioUtils shall
load the image list from glance only if glance is available.
Otherwise the test discovery process will fail if glance
is not available.

Closes bug 1277669

Change-Id: I4be7b9b88d9f8ff42059ece84a3c58df4fb90f1e
Merge "InputScenarioUtils load images if glance available"