skip if engine already has overlay network plugin
skip if engine already has overlay network plugin

Signed-off-by: allencloud <36603fc76d0c5a59db517817381acc1be27b9499@daocloud.io>

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