Rework class inheritance for scenario tests

This commit reworks the class inheritance used by the scenario tests
to make is a bit simpler and similar to what is used for the api
tests. The scenario tests now inheret from the base test case directly
and client creation is handled directly instead of looping over the
client_attr_names list.

Change-Id: Id8bf2db3ec8f706ca466b86fa71df3ee2607fe88
Merge "Rework class inheritance for scenario tests"