add graphite logo to datasource card

public/app/plugins/datasource/graphite/plugin.json

@@ -14,6 +14,10 @@
     "author": {
       "name": "Grafana Project",
       "url": "http://grafana.org"
+    },
+    "logos": {
+      "small": "img/graphite_logo.png",
+      "large": "img/graphite_logo.png"
     }
   }
-}
+}