Resolve TODO in test: 'test_available_volume_retype_with_migration'

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

Change-Id: I6588c3344ade0d47899753619dc1f9beec4bb4b4
diff --git a/tempest/api/volume/admin/test_volume_retype_with_migration.py b/tempest/api/volume/admin/test_volume_retype_with_migration.py
index 94d5299..f0b3a4f 100644
--- a/tempest/api/volume/admin/test_volume_retype_with_migration.py
+++ b/tempest/api/volume/admin/test_volume_retype_with_migration.py
@@ -85,9 +85,7 @@
         volume_source = self.admin_volume_client.show_volume(
             self.src_vol['id'])['volume']
 
-        # TODO(erlon): change this to volumes_client client after Bug
-        # #1657806 is fixed
-        self.admin_volume_client.retype_volume(
+        self.volumes_client.retype_volume(
             self.src_vol['id'],
             new_type=self.dst_vol_type['name'],
             migration_policy='on-demand')