[rackspace|identity] Correctly populate model from request data.

Previously, the model was being incorrectly loaded from request data. As a
result, the username and apiKey properties were always nil. This fixes parsing
and improves the tests to catch this type of bug in the future.
[rackspace|identity] Remove unused get_credentials request.
Merge pull request #1527 from bradgignac/rackspace-identity-credentials

[rackspace|identity] Fix credential parsing.