fix for latest report in overview

* report/latest/<node_name> uses limit parameter in _query
  to get just one report
* disable "Latest Report" button if there is no report
* HTTP Status 500 if there is no report on report/latest/<node_name>

puppetboard/app.py

@@ -203,11 +203,15 @@ def report_latest(node_name):
     as long as PuppetDB can't filter reports for latest-report? field. This
     feature has been requested: http://projects.puppetlabs.com/issues/21554
     """
-    # TODO: use limit parameter in _query to get just one report
     node = get_or_abort(puppetdb.node, node_name)
-    reports = ten_reports(node.reports())
-    report = list(yield_or_stop(reports))[0]
-    return redirect(url_for('report', node=node_name, report_id=report))
+    reports = get_or_abort(puppetdb._query, 'reports',
+                           query='["=","certname","' + node_name + '"]',
+                           limit=1)
+    if len(reports) > 0:
+        report = reports[0]['hash']
+        return redirect(url_for('report', node=node_name, report_id=report))
+    else:
+        abort(500)
 
 
 @app.route('/report/<node>/<report_id>')