Set node name as prometheus instance name

And DRY up the recipe so it's easy and expressive to add more
metrics/targets

site-cookbooks/kosmos_prometheus/templates/default/prometheus.yml.erb

@@ -14,11 +14,13 @@ scrape_configs:
 <% end %>
   metrics_path: "<%= job.fetch('metrics_path', '/metrics') %>"
   static_configs:
-    - targets: <%= Array(job['targets']) %>
-<% if job['labels'] %>
+<% job['targets'].each do |t| %>
+    - targets:
+        - <%= t['target'] %>
       labels:
-<% job['labels'].each do |label, label_config| %>
-        <%= label %>: <%= label_config %>
+        instance: <%= t['instance'] %>
+<% if t['env'] %>
+        env: <%= t['env'] %>
 <% end %>
 <% end %>
 <% end %>