agent: Quiet warning about executor support for secrets
agent: Quiet warning about executor support for secrets

The agent will warn about the executor not supporting secrets whenever
it receives an assignments update from the dispatcher. This adds a lot
of noise to logs from things like integration tests.

Instead of warning every time, we should only warn when the assignment
set contains at least one secret addition or removal.

Signed-off-by: Aaron Lehmann <8ecfc6017a87905413dcd7d63696a2a4c351b604@docker.com>

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