torture: Forgive non-plural arguments

This commit allows --bootarg instead of --bootargs, --config instead of
--configs, and --qemu-arg instead of --qemu-args.  For those cases where
a native English speaker might auto-correct the argument to be incorrect.

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