Use newfeed.service config for all newsfeeds

Prior to this change, the main `kibana` newsfeed would respect the
`newsfeed.service.urlRoot` config setting, but other feeds like
`kibana-analytics` would not. This made it difficult to test new feed
items using a local server.

src/plugins/newsfeed/public/plugin.tsx

@@ -51,7 +51,10 @@ export class NewsfeedPublicPlugin
     return {
       createNewsFeed$: (endpoint: NewsfeedApiEndpoint) => {
         const config = Object.assign({}, this.config, {
-          service: { pathTemplate: `/${endpoint}/v{VERSION}.json` },
+          service: {
+            pathTemplate: `/${endpoint}/v{VERSION}.json`,
+            urlRoot: this.config.service.urlRoot,
+          },
         });
         return this.fetchNewsfeed(core, config);
       },