Set swift_operator_role if required

Set swift_operator_role if required

Apex creates member as default role which conflicts with the default
swift_operator_role [1]. It detects the default role in lowercase as
well and write it in rally.conf if required.

[1] https://build.opnfv.org/ci/view/functest/job/functest-apex-virtual-suite-master/116/console

Change-Id: Ic4dc98c793207fc32ff35852472742c92db6699c
Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>
(cherry picked from commit bafbd217c68c70ae66e052833c3e33298b35da0c)