Fix mongodb_user roles bug

```
2014-06-21 15:54:31 +0200 Puppet (err): Execution of '/usr/bin/mongo app --quiet --host maintenance-router.mongodb.app.com:27017 --eval db.updateUser("app", {"roles":["dbAdmin"]})' returned 252: 2014-06-21T08:54:31.582-0500 Error: Updating user failed: Could not lock auth data update lock. at src/mongo/shell/db.js:1105
2014-06-21 15:54:31 +0200 /Stage[main]/App::Mongodb::Schema/Mongodb_user[app]/roles (err): change from [{"role"=>"dbAdmin", "db"=>"app"}] to ["dbAdmin"] failed: Execution of '/usr/bin/mongo app --quiet --host maintenance-router.mongodb.app.com:27017 --eval db.updateUser("app", {"roles":["dbAdmin"]})' returned 252: 2014-06-21T08:54:31.582-0500 Error: Updating user failed: Could not lock auth data update lock. at src/mongo/shell/db.js:1105
```

Seems to continuously apply this change.