This change moves all of the fedora28 container images to centos8 to ensure
we're maximizing coverage in all of our roles with OS images that represent
real deployments. While fedora28 was useful to exercise RHEL8 code path's
while there was no RHEL/CentOS8 container image, we can now use the upstream
CentOS8 image ensuring we're getting a true, representative, environment.
Signed-off-by: Gael Chamoulaud (Strider) <firstname.lastname@example.org>