torture: Allow "CFLIST" to specify default list of scenarios

On a large system, it can be convenient to tell rcutorture to run
several instances of the default scenarios.  Currently, this requires
explicitly listing them, for example, "--configs '2*SRCU-N 2*SRCU-P...'".
Although this works, it is rather inconvenient.

This commit therefore allows "CFLIST" to be specified to indicate the
default list of scenarios called out in the relevant CFLIST file, for
example, for RCU, tools/testing/selftests/rcutorture/configs/rcu/CFLIST.
In addition, multipliers may be used to run multiple instances of all
the scenarios.  For example, on a 256-CPU system, "--configs '3*CFLIST'"
would run three instances of each scenario concurrently with one CPU
left over.  Thus "--configs '3*CFLIST TINY01'" would exactly consume all
256 CPUs, which makes rcutorture's jitter feature more effective.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>