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.