raft: check if the last node can still process updates in demote test
raft: check if the last node can still process updates in demote test

Signed-off-by: Alexandre Beslic <7f00c88b7660208d6ae438e5aa377b1c70b9009e@gmail.com>

aaronlehmann/swarmkit,elifa/swarmkit,fate-grand-order/swarmkit,deljuven/swarmkit,elifa/swarmkit,docker/swarmkit,docker/swarmkit,vasil-yordanov/swarmkit,cezarsa/swarmkit,cheyang/swarmkit,yongtang/swarmkit,yp-engineering/swarmkit,deljuven/swarmkit,Noyan759/swarmkit,allencloud/swarmkit,Noyan759/swarmkit,RenaudWasTaken/swarmkit,diogomonica/swarmkit,jlhawn/swarmkit,AliyunContainerService/swarmkit,dperny/swarmkit-1,fate-grand-order/swarmkit,AliyunContainerService/swarmkit,yongtang/swarmkit,aaronlehmann/swarmkit,allencloud/swarmkit,RenaudWasTaken/swarmkit,alyyousuf7/swarmkit,diogomonica/swarmkit,jlhawn/swarmkit,sungwonh/swarmkit,dperny/swarmkit-1,sungwonh/swarmkit,alyyousuf7/swarmkit,yp-engineering/swarmkit,cheyang/swarmkit,cezarsa/swarmkit,vasil-yordanov/swarmkit