rcutorture: Enable slow grace-period initializations

This commit sets CONFIG_RCU_TORTURE_TEST_SLOW_INIT=y, but leaves the
default time zero.  This can be overridden by passing the
"--bootargs rcutree.gp_init_delay=1" argument to kvm.sh.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>