The image download tests have long been too unreliable for the gate.
We need to cache the base images similar to how devstack caches it's
testing images. Let's move them to non-voting jobs for the time
being.
This means that the gate jobs are now all based on "-minimal" and are
using infra mirrors. Unfortunately, there is still some unreliability
because we currently have issues with infra mirrors being very slow
after AFS updates, leading to job timeouts. But we're on the right
path...
Also, I noticed we don't have tests of the "ubuntu" image-download
based tests, which were tacitly being tested by apt-sources before we
moved that to -minimal. Add simple tests for these.
Change-Id: Ie33ee49656872467ef68d753210032156bb6b2cb
b0e0dd991c