Use `is_tag` for DeploymentTracker logger payload

This resolves an Elasticsearch issue where sometimes payload.tag was a Boolean (as here), and sometimes it was a String (in other locations). This renames the payload key used here to is_tag, which is consistent with the parameter to the create_deployments method in this class.

This should resolve the immediate problem of gitlab-com/gl-infra/delivery#1325, however should not close that issue as we need a more holistic solution for dealing with differing payload value types in Elasticsearch.