engine: placeholders of child commands aren't cleared when failing during the CDA phase

AsyncTask placeholders for child commands are not
cleared when canDoAction returns false.

Change-Id: Ie77a00b0071c92d074d837768af012c80374169f
Bug-Url: https://bugzilla.redhat.com/1139678
Signed-off-by: Ravi Nori <rnori@redhat.com>