Change Compute default namespacing

Allow for Fog::<provider>::Compute as well as Fog::Compute::<provider>.
Generic solution so as to avoid more hard-coding in compute.rb.

Includes spec based on identity_spec.rb though repurposed for Compute.

For 2.0, the Fog::<provider>::<service>/Fog::<service>::<provider>
should be replaced with more explicit names.  Perhaps
Fog::Builtins::<Provider>::<Service> and
Fog::Extensions::<Provider>::Service or similar.

TL;DR: I'd rather have another layer of modules, containing an intent-
revealing name than a conventon where Fog::<provider>::<service> implies
one thing and Fog::<service>::provider implies another.