TestNodeRejoins tests that both a manager and agent will fail early if the certificates on disk have been changed.  This should be the case for agent, but not necessarily the case for manager, so remove that assumption from the test.
TestNodeRejoins tests that both a manager and agent will fail early if the
certificates on disk have been changed.  This should be the case for agent,
but not necessarily the case for manager, so remove that assumption from
the test.

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

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