Adjust authentication.rst doc to reference "identity_uri"

The "auth_port", "auth_host", and "auth_protocol" variables were
deprecated in favour of a single "identity_uri" variable.

* Adjust authentication.rst doc to reference "identity_uri"

Change-Id: I48de53f21b8d767b276858ed274066015d765f0e
Closes-Bug: #1361613