Remove image downloading from Murano test

If image for Murano tests doesn't exist on target
host test tries to download it from non-existing
URL. Image should be downloaded manually before
running tests as specified in docs. Also fixed
checking of OSTF tests results.

Change-Id: I52cc93fc111c4e4284f8ac43b5035b38925f2374
Closes-bug: #1327290
(cherry picked from commit 654abb0c2ca0175ecdfc05796c97a29c6b13afca)