Fixes image download script

Fixes image download script

Fixes Include:
 - Fix previous behavior where multiple copies of the same file would be
   downloaded as blah.qcow.1, blah.qcow.2 and not overwrite original
   file.
 - Fix issue where files were always downloaded, even when they were not
   different from server version.

It temporarily stops downloading from marketplace.openbaton.org:8082
which is blocked by LF firewalls.
wget options are tuned to avoid wasting jjobs time.

Change-Id: I9191847a3f80045329c160e86655cc0a0e80831c
Signed-off-by: Tim Rozet <trozet@redhat.com>