Fix for Authentication scenario to correctly use self.clients

Scenario has recently been refactored, self.clients in Scenario
now takes the name of the CLI client. During the refactoring,
the Authenticate scenario was not correctly updated, which
causes the authentication scenario to fail. This patch fixes
that.

Change-Id: I546c0846e00a5285f0d47bc80b6304a53cc566ff
Closes-Bug: #1291386