puppetboard/app.py: Protecting against ZeroDivisionErrors on calculations (#244)

This (re-)fixes https://github.com/voxpupuli/puppetboard/issues/220

Corner cases where new installations or empty environments will return
zero resources and zero nodes, this will result in a ZeroDivisionError.
Now if this happens setting the average-resources-per-node to zero.

puppetboard/app.py

@@ -174,8 +174,11 @@ def index(env):
                     env))
         metrics['num_nodes'] = num_nodes[0]['count']
         metrics['num_resources'] = num_resources[0]['count']
+        try:
             metrics['avg_resources_node'] = "{0:10.0f}".format(
                 (num_resources[0]['count'] / num_nodes[0]['count']))
+        except ZeroDivisionError:
+            metrics['avg_resources_node'] = 0
 
     nodes = get_or_abort(puppetdb.nodes,
         query=query,