Ensure that the test CA, if an external CA is desired, does not have a local signer.
Ensure that the test CA, if an external CA is desired, does not have a local signer.

Signed-off-by: Ying Li <f39392f1e2d21dabc2bc5ab11d2bf832bde87b60@docker.com>

AliyunContainerService/swarmkit,Noyan759/swarmkit,docker/swarmkit,diogomonica/swarmkit,Noyan759/swarmkit,dperny/swarmkit-1,sungwonh/swarmkit,jlhawn/swarmkit,jlhawn/swarmkit,AliyunContainerService/swarmkit,RenaudWasTaken/swarmkit,diogomonica/swarmkit,RenaudWasTaken/swarmkit,cheyang/swarmkit,alyyousuf7/swarmkit,alyyousuf7/swarmkit,dperny/swarmkit-1,sungwonh/swarmkit,cheyang/swarmkit,docker/swarmkit