swarmctl: add a validation for 'swarmctl version'
swarmctl: add a validation for 'swarmctl version'

Signed-off-by: Kei Ohmura <82a3f76f5859104098e1c763aea3695df488375c@gmail.com>

diogomonica/swarmkit,RenaudWasTaken/swarmkit,fate-grand-order/swarmkit,AliyunContainerService/swarmkit,AliyunContainerService/swarmkit,fate-grand-order/swarmkit,docker/swarmkit,allencloud/swarmkit,yongtang/swarmkit,dperny/swarmkit-1,yongtang/swarmkit,dperny/swarmkit-1,elifa/swarmkit,deljuven/swarmkit,diogomonica/swarmkit,sungwonh/swarmkit,RenaudWasTaken/swarmkit,Noyan759/swarmkit,cheyang/swarmkit,alyyousuf7/swarmkit,allencloud/swarmkit,sungwonh/swarmkit,jlhawn/swarmkit,elifa/swarmkit,cheyang/swarmkit,Noyan759/swarmkit,docker/swarmkit,alyyousuf7/swarmkit,deljuven/swarmkit,jlhawn/swarmkit