rcutorture: Exchange TREE03 and TREE08 NR_CPUS, speed up CPU hotplug

TREE03 has been especially effective at finding bugs lately.  This commit
makes it even more effective by speeding up its CPU hotplug testing and
increasing its NR_CPUs from 8 to 16.  TREE08's NR_CPUS is decreased from
16 to 8 in order to maintain the same test duration.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>