Add ForceUpdate to TaskSpec

[naku]

The ForceUpdate parameter is missing from TaskSpec. It'd be useful to rebalance a service.

From the doc : ForceUpdate is a counter that triggers an update even if no relevant parameters have been changed

This should be equivalent to the command line :
docker service update --force service_name

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[davidxia]

Thanks for reporting. PRs welcome.

[dmandalidis]

Hi @naku ,

Since this project went on mature status, please re-open this issue (if it still stands) to https://github.com/dmandalidis/docker-client. Thanks