qa: stop overriding distro for k-testing

qa: stop overriding distro for k-testing

This is a continuation of previous commit

    qa: only use RHEL for workload testing

We don't want to test fs:workload with centos/ubuntu to avoid packaging
issues and to reduce the matrix of distros we're running workloads on.
Also, the testing kernel should install fine on the distros we test with
"supported" random distros.

Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>