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