Added support for domain based authentication for Rackspace

### Changes

- Added support for authentication against the Rackspace Cloud using the __RAX-AUTH:domain__ extension
-  Moved the `RackspaceCloudIdentity` into the __Providers->Rackspace->Objects__ namespace.
- Added tests to verify functionality.  Requires a new test identity be added to the .openstack_net test file *(Note: You will need to fill in the values below with your credentials)*:

    ```
"TestDomainIdentity" : {
		"Username" : "",
		"Password" : "",
		"APIKey" : "",
		"TenantId" : "",
		"Domain" : ""
	},

    ```