qa: samba tests need to run on testing kernel
I think some of our "random" samba failures are not so random. It looks like we no longer pass on any of our kclient mount tests, and when I look through the logs, they're actually getting an IO error on mount!
This actually makes some sense: the default kernels we're using (at least on ubuntu) are too old to mount a default-config CephFS any more (wrong tunables). We could configure the cluster to use old tunables, but since the whole point is to test Samba-on-CephFS, I think we want to run it against a testing kernel.