Update 'Member' with option 'operator_role' in tempest.conf

In function _get_object_storage_client(), it is better to use the
option 'operator_role' in tempest.conf than hard code the role
to 'Member' here.

Change-Id: Id1de7ee55159f6583a0a93ee87e26a5deaacfd6e
Closes-Bug: #1267857
(cherry picked from commit 5384ef569b36abaa8078f4114ec0187d9280f47b)