Return npm site location as artifact

Later, we will update zuul-preview to look up artifact URLs for
sites and return that as the success url.

Change-Id: I8eae92e0b94085241a30b7d6e9b2894e1d07096e

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

@@ -140,3 +140,11 @@
     src: "{{ coverage_output_src }}"
     verify_host: true
   when: coverage_report_stat.stat.exists
+
+- name: Return site artifact location to Zuul
+  zuul_return:
+    data:
+      zuul:
+        artifacts:
+          - name: "site"
+            url: "npm/html/"