Use addCleanup to detach volume instead of explicit detach

Cinder volume delete [1] is executed during resource cleanup.
So, if test fails due to some reason then it tries to delete a volume
which is "in-use" state. Detach volume should be executed before
trying to delete the volume. This patch adds nova_detach volume
as a cleanup task.

[1] https://github.com/openstack/barbican-tempest-plugin/blob/a6d945d8e677dedf73ccb7b2f7271e8fc96836be/barbican_tempest_plugin/tests/scenario/manager.py#L229

Change-Id: I3457bb6fc22a7bb6a32c8d6f4ea5879aeb8d43c5
Related-Bug: #1755485
diff --git a/barbican_tempest_plugin/tests/scenario/manager.py b/barbican_tempest_plugin/tests/scenario/manager.py
index 1aa009e..66d28a5 100644
--- a/barbican_tempest_plugin/tests/scenario/manager.py
+++ b/barbican_tempest_plugin/tests/scenario/manager.py
@@ -333,6 +333,7 @@
         waiters.wait_for_volume_resource_status(self.volumes_client,
                                                 volume['id'], 'in-use')
 
+        self.addCleanup(self.nova_volume_detach, server, volume)
         # Return the updated volume after the attachment
         return self.volumes_client.show_volume(volume['id'])['volume']