As discussed on opensuse-kernel ML, there is a request to switch the kernel
default config for preemption to CONFIG_PREEMPT_VOLUNTARY from the current
CONFIG_PREEMPT on Tumbleweed:
  https://lists.opensuse.org/opensuse-kernel/2019-02/msg00005.html

Originally, this theme was raised from another bug 1112824, where it indicated
that CONFIG_PREEMPT_VOLUNTARY works around the GNOME performance problem.

Although switching the preemption behavior shouldn't be taken as the fix for
the problem above, we can consider it as a good chance to switch.
As found in the discussion thread above, other major distros (Fedora and
Ubuntu) use CONFIG_PREEMPT_VOLUNTARY as default.  So this should be good enough
for the interactivity we wanted to achieve with CONFIG_PREEMPT, while it must
be better in terms of the throughput.

Moreover, the current TW kernel config has very inconsistency; only a few
(x86_64/default, armv7hl/default and lpae, and i386/pae) take CONFIG_PREEMPT=y
while others take CONFIG_PREEMPT_NONE=y.  This inconsistency likely came from
the fact that SLE and Leap kernels take CONFIG_PREEMPT_NONE=y for all flavors.
So, we should change all to CONFIG_PREEMPT_VOLUNTARY=y for all flavors on TW
from consistency POV, too.

Also, it'd be a good timing to switch the Leap 15.1 config as well, so that we
can align the behavior with TW kernel.