Fixed get_auth_info() for pre-authenticated clients

When a client is instantiated with pre-defined authorization token,
no authentication is done, so no auth_[tenant|user]_id keys are set
for HTTPClient. As a result, Client.get_auth_info() fails with KeyError.

Fixed the issue by setting those keys on HTTPClient initialization.

Change-Id: Ifc9d6b128286b635346a7bc026fcc55dbd002050
Closes-Bug: 1277120