Use a non admin privileges for retyping a volume

Retype of a volume does not require admin privileges,
since Bug #1657806 was fixed.

Change-Id: Ia4a9bd20af0fcc5991ea45c2fd2a78f417d67d72
diff --git a/tempest/scenario/test_volume_migrate_attached.py b/tempest/scenario/test_volume_migrate_attached.py
index 5667fbb..6cdaa23 100644
--- a/tempest/scenario/test_volume_migrate_attached.py
+++ b/tempest/scenario/test_volume_migrate_attached.py
@@ -38,11 +38,6 @@
     credentials = ['primary', 'admin']
 
     @classmethod
-    def setup_clients(cls):
-        super(TestVolumeMigrateRetypeAttached, cls).setup_clients()
-        cls.admin_volumes_client = cls.os_admin.volumes_v2_client
-
-    @classmethod
     def skip_checks(cls):
         super(TestVolumeMigrateRetypeAttached, cls).skip_checks()
         if not CONF.volume_feature_enabled.multi_backend:
@@ -82,7 +77,7 @@
 
     def _volume_retype_with_migration(self, volume_id, new_volume_type):
         migration_policy = 'on-demand'
-        self.admin_volumes_client.retype_volume(
+        self.volumes_client.retype_volume(
             volume_id, new_type=new_volume_type,
             migration_policy=migration_policy)
         waiters.wait_for_volume_retype(self.volumes_client,