Fix tempest-account-generator command roles issue

The change https://review.openstack.org/#/c/216109 modified the
identity client to return the entire response object instead of
only part of it. This patch fixes a call to list_roles that was
missed in the tempest-account-generator tool.

Change-Id: I0986dac6f8a37879f98583f2e1c899670970b668
diff --git a/tempest/cmd/account_generator.py b/tempest/cmd/account_generator.py
index c26df96..02c6e7f 100755
--- a/tempest/cmd/account_generator.py
+++ b/tempest/cmd/account_generator.py
@@ -160,7 +160,7 @@
 def create_resources(opts, resources):
     (identity_admin, neutron_iso_networks,
      network_admin, networks_admin) = get_admin_clients(opts)
-    roles = identity_admin.list_roles()
+    roles = identity_admin.list_roles()['roles']
     for u in resources['users']:
         u['role_ids'] = []
         for r in u.get('roles', ()):