Allow for bad Fedora image mirrors

I am getting a lot of http 404's from Fedora mirrors, in particular in
the US. Alternatively we could get cache-url to use previously existing
files if they exist and there is an error but this may not be the
correct thing todo in all cases.

So for now we do a retry if HTTP 404 is returned to the request for a
Fedora image.

Change-Id: I7634aeac3bdfe6930fafe6d276186c09fc8587b5
Merge "Allow for bad Fedora image mirrors"