It's a good practice to always have the same components used in tests.
According to:

   https://cloud-images.ubuntu.com/releases/16.04/



New images are released from time to time, and the "release/"
directory points to the latest release.  Let's pin to the latest
available version, and while at it, set a hash for verification.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190613130718.3763-4-crosa@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>