[workflows] Workflow definitions not refreshed after upgrade

When upgrading to a new version of Mender, there is a potential problem with workflows when starting the backend for the first time: since workflows-server starts before the workflows-worker the definitions are updated after workflows-server has loaded the old definitions in memory. As a result, when the new definitions breaks the http schema, the workflow stop working entirely (until the user restart the workflows-server) as seen in this thread.

To prevent this from happening, the when the workflows are loaded from the file, (all) server instances should refresh the cached definitions.