Show Created At column for volume backups

The ``volume backup list`` command does not show the
created_at column for backups which doesn't really
help the end-user since a critical part of a backup
is knowing when it was taken, and fast if there is
a rush doing recovery by restoring a volume from a
backup.

Change-Id: I8d8a7f36c468c9faa2c2c47bfa9ba9e1ca5b9858

openstackclient/releasenotes/notes/volume-backup-created-at-list-b49ec893ae1f6b0d.yaml

@@ -0,0 +1,4 @@
+---
+features:
+  - |
+    Listing volume backups now shows the created_at column.

openstackclient/tests/unit/volume/v2/test_volume_backup.py

@@ -359,6 +359,7 @@ class TestBackupList(TestBackup):
         'Status',
         'Size',
         'Incremental',
+        'Created At',
     )
     columns_long = columns + (
         'Availability Zone',
@@ -376,6 +377,7 @@ class TestBackupList(TestBackup):
                 b.status,
                 b.size,
                 b.is_incremental,
+                b.created_at,
             )
         )
     data_long = []
@@ -388,6 +390,7 @@ class TestBackupList(TestBackup):
                 b.status,
                 b.size,
                 b.is_incremental,
+                b.created_at,
                 b.availability_zone,
                 volume_backup.VolumeIdColumn(b.volume_id),
                 b.container,

openstackclient/volume/v2/volume_backup.py

@@ -305,6 +305,7 @@ class ListVolumeBackup(command.Lister):
             'status',
             'size',
             'is_incremental',
+            'created_at',
         )
         column_headers = (
             'ID',
@@ -313,6 +314,7 @@ class ListVolumeBackup(command.Lister):
             'Status',
             'Size',
             'Incremental',
+            'Created At',
         )
         if parsed_args.long:
             columns += ('availability_zone', 'volume_id', 'container')