Identity versioning & discovery refactor

**** REVIEW ONLY ****
We are attempting to implement a couple of features with this commit:
1) Split up he Identity class into two separate classes, one each for v2 and v3 of the Identity service. We will also move all files (requests, specs, etc.) associated with the specific service (ex: Identity v2) into an easily identifiable folder structure by version.
2) Implement a very basic discovery class/mechanism to allow the app developer to hit OpenStack via the fog gem without having to know what versions of a specific service are available.