Correct policy action for backup export volume endpoint

This PS corrects the policy action for exporting a volume
backup from "backup:backup-export" to "backup:export-import".
The former is causing the related RBAC test to fail [0] and
the in-code policy documentation confirms that the latter is
correct [1].

[0] http://logs.openstack.org/08/518208/3/check/legacy-tempest-dsvm-patrole-multinode-admin/5f129df/job-output.txt.gz#_2017-11-08_01_02_55_782843
[1] https://github.com/openstack/cinder/blob/fb27334719fb612d2d5386b7d9de374d4a415d81/cinder/policies/backups.py#L127

Change-Id: I10dd9cb8134a2b83cf92131c1b28a58881757f32
diff --git a/patrole_tempest_plugin/tests/api/volume/test_volumes_backup_rbac.py b/patrole_tempest_plugin/tests/api/volume/test_volumes_backup_rbac.py
index 7ca3d9f..e461723 100644
--- a/patrole_tempest_plugin/tests/api/volume/test_volumes_backup_rbac.py
+++ b/patrole_tempest_plugin/tests/api/volume/test_volumes_backup_rbac.py
@@ -141,7 +141,7 @@
 
     @decorators.attr(type='slow')
     @rbac_rule_validation.action(service="cinder",
-                                 rule="backup:backup-export")
+                                 rule="backup:export-import")
     @decorators.idempotent_id('e984ec8d-e8eb-485c-98bc-f1856020303c')
     def test_export_backup(self):
         backup = self.create_backup(volume_id=self.volume['id'])