Set heat stack role in tempest config to _member_

In kilo heat started to use keystone delegations to perform the needed
operations, as part of this the need to set the explicit role in
devstack for stack management disappeared. However, in tempest as part
of the effort to make credentials configuration more explicit an option
was added to ensure that the users created by tempest have the proper
role set for stack management in the heat tests. This commit sets the
value of this config option in tempest to be the default role _member_
to reflect that there is no separate heat_stack_owner role created
anymore. (which is the tempest default value)

Change-Id: Id98a83f0a716de0fdb5f36d03407364830e8fa5f
diff --git a/lib/tempest b/lib/tempest
index 9b44f47..5f7be3c 100644
--- a/lib/tempest
+++ b/lib/tempest
@@ -399,6 +399,7 @@
         fi
         iniset $TEMPEST_CONFIG orchestration instance_type "m1.heat"
         iniset $TEMPEST_CONFIG orchestration build_timeout 900
+        iniset $TEMPEST_CONFIG orchestration stack_owner_role "_member_"
     fi
 
     # Scenario