qa: avoid qa/workunits/cephtool/test.sh unstability

For testing injectargs a configuration option was changed that has side
effects on the cluster. It could introduce random failures later. It is
replaced with a configuration option that cannot have adverse side
effects on the cluster.

http://tracker.ceph.com/issues/9919 Fixes: #9919

Signed-off-by: Loic Dachary <loic-201408@dachary.org>