Add PASSWORD_SEED env to app.json

app.json

@@ -17,6 +17,10 @@
       "description": "A secret key for verifying the integrity of signed cookies.",
       "generator": "secret"
     }
+    "PASSWORD_SEED": {
+      "description": "A password seed is used by password hash generator. ",
+      "generator": "secret"
+    }
   },
   "addons": [
     "mongohq"