burnin: only remove instances we actually added

Currently burnin, if proceding in parallel, will remove all instances
which were passed, even if they failed to add. This is bad because it
will also remove instances which existed before burnin started. By
adding the instances to the removal queue only if their creation was
successful (passing the action as a post processing action to
ExecOrQueue) we guarantee pre-existing instances are saved.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>