Merge pull request #2203 from mozilla/env-var-for-kpi-url

can configure kpi datastore url with env var

Logically the same as what was previously there.  

r+

lib/configuration.js

@@ -138,7 +138,11 @@ var conf = module.exports = convict({
     format: 'number = 0.0',
     env: 'KPI_BACKEND_SAMPLE_RATE'
   },
-  kpi_backend_db_url: 'string = "http://localhost/wsapi/interaction_data"',
+  kpi_backend_db_url: {
+    doc: "URL of KPiggyBank service to send Key Performance Indicator data to",
+    format: 'string = "http://localhost/wsapi/interaction_data"',
+    env: 'KPI_BACKEND_DB_URL'
+  },
   bcrypt_work_factor: {
     doc: "How expensive should we make password checks (to mitigate brute force attacks) ?  Each increment is 2x the cost.",
     format: 'integer{6,20} = 12',