Use Rails environments instead of DB configurations

In the admin forms for `Spree::PaymentMethod` and `Spree::Tracker` the collection used for the environment field is the database configuration keys. This is wrong and it becomes quite apparent in the case of applications that connect to more than one database. This PR replaces said collection with a collection of the available environment configurations in '/config/environments/'.

The methods that validate the `environment` attribute for these classes already compare them against `Rails.env`, so no changes are needed there.
Will do.