tests: update Fedora i386 cross image to Fedora 29

Using the "latest" tag is not a good idea because this changes what
release it points to every 6 months. Together with caching of docker
builds this can cause confusion where CI has cached & built with Fedora
N, while a developer tries to reproduce a CI problem with Fedora N + 1,
or vica-verca.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>