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
diff --git a/lib/tempest b/lib/tempest
index 60f571c..a0f9dd8 100644
--- a/lib/tempest
+++ b/lib/tempest
@@ -581,7 +581,7 @@
     tox -evenv-tempest -- pip install -c u-c-m.txt -r requirements.txt
 
     # Auth:
-    iniset $TEMPEST_CONFIG auth tempest_roles "Member"
+    iniset $TEMPEST_CONFIG auth tempest_roles "member"
     if [[ $TEMPEST_USE_TEST_ACCOUNTS == "True" ]]; then
         if [[ $TEMPEST_HAS_ADMIN == "True" ]]; then
             tox -evenv-tempest -- tempest-account-generator -c $TEMPEST_CONFIG --os-username $admin_username --os-password "$password" --os-tenant-name $admin_project_name -r $TEMPEST_CONCURRENCY --with-admin etc/accounts.yaml