fixes a nasty bug for which it wasn't possible to use this gem in a multi-user environment.
Explanation:
The ActiveReSource:Base::prefix method is created at runtime (for every subclass of AcrtiveResource::Base) in order to deal with nested resource. These dynamic methods relay on a "fixed" site URI.
Since OpenStack API uses the tenant id in the prefix for a given resource, first time the ActiveReSource:Base::prefix= method was called a prefix method was created with the tenant_id used.