Use identity type rather than defaulting to rackspace.

Changes made in ff15bcce were causing auth to break. Before ff15bcce the auth plugin was always getting passed into the novaclient's init because the default_identity_type was always set and:

```python
if default_identity_type and default_identity_type != "keystone":
    auth_plugin = _cs_auth_plugin.load_plugin(default_identity_type)
```
would cause auth_plugin to always get passed in. Now when using a keystone identity type, the auth_plugin is getting passed in as None, and when combined with an auth_system type of rackspace, causes the novaclient to raise: AuthSystemNotFound: AuthSystemNotFound: 'rackspace'