fetch-javascript-output: add metadata.type

This adds a 'site' metadata type to the role so that we can pull
it down later based on type instead of a user friendly "name"

Change-Id: I4c59feac26971162a29fb998a21c422ccb4efc1a

roles/fetch-javascript-output/tasks/main.yaml

@@ -148,3 +148,5 @@
         artifacts:
           - name: "Site preview"
             url: "npm/html/"
+            metadata:
+              type: site