Rename "Member" role in tempest_roles to "member"

Recently, Keystone renamed "Member" role to "member"
(case-sensitive) with https://review.openstack.org/#/c/572243/14

Case-sensitivity role requirement in Keystone was recently
formalized with https://review.openstack.org/#/c/576640/

From the above reference:

"Role names are case-insensitive. for example, when keystone
bootstraps default roles, it creates `admin`, `member`, and
`reader`. If another role `Member` (note the upper case 'M') is
created, keystone will return a `409` Conflict since it considers
the name "Member" == "member". Note that case is preserved in these
cases."

It follows that Tempest should use "member" role by default.

Change-Id: Iebf04fdb4c195b6779c74f66da3f7822cf174494
1 file changed