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.

Closes-Bug: 1277120
Closes-rally-bug: DE726
Implements: rally-task TA4473
Upstream-Review: https://review.openstack.org/#/c/71551
Not-in-upstream: false
Merge pull request #6 from CiscoSystems/de726

Fixed get_auth_info() for pre-authenticated clients