catch DriverLoadFailure for get_transport optional

new oslo.messaging 5.2.0 release will raise
oslo_messaging.transport.DriverLoadFailure if no existent transport
driver, hence breaks our code and unit test as well.

This patch fixes it by adding DriverLoadFailure to catch list.

Also:

tempest: import ImagesClient as ImagesClientV2

This commit [1] renamed ImagesClientV2 to ImagesClient so Ceilometer
Tempest tests do not work anymore.

[1] https://github.com/openstack/tempest/commit/2fea405bd6ea8674c8daaf8ebb788ae33d52c976

Change-Id: I02b5d1b3f4a9169cbf428a5eff9c6705494741af
Closes-Bug: #1588151
(cherry picked from commit d7f8ce94e433da8af3660ecf52715a04c221fde5)
(cherry picked from commit f6b2406fffdd4deae508b9f251691fe9b70cb1ee)
diff --git a/ceilometer/tests/tempest/service/client.py b/ceilometer/tests/tempest/service/client.py
index 81d67d8..dedebc7 100644
--- a/ceilometer/tests/tempest/service/client.py
+++ b/ceilometer/tests/tempest/service/client.py
@@ -24,7 +24,8 @@
 from tempest.lib.services.compute.servers_client import ServersClient
 from tempest import manager
 from tempest.services.image.v1.json.images_client import ImagesClient
-from tempest.services.image.v2.json.images_client import ImagesClientV2
+from tempest.services.image.v2.json.images_client import (
+    ImagesClient as ImagesClientV2)
 from tempest.services.object_storage.container_client import ContainerClient
 from tempest.services.object_storage.object_client import ObjectClient