add test for create_backup

*add positive test and corresponding client for server
action: createBackup.
*verify that create backup successfully and rotate
backups correctly.
*add the property prefix filter support in image_list_detail
of the image service client, because the image api in nova can't
support sort_dir='asc' and sort_key='created_at'.
so we can submit request like the following to get specified images

GET /images/detail?sort_key=created_at&property-image_type=backup

Change-Id: I43337970e4b1b71c7290416c517a8f93152b168f
diff --git a/tempest/services/compute/xml/servers_client.py b/tempest/services/compute/xml/servers_client.py
index 43de4ef..7b05138 100644
--- a/tempest/services/compute/xml/servers_client.py
+++ b/tempest/services/compute/xml/servers_client.py
@@ -411,6 +411,13 @@
             body = xml_to_json(etree.fromstring(body))
         return resp, body
 
+    def create_backup(self, server_id, backup_type, rotation, name):
+        """Backup a server instance."""
+        return self.action(server_id, "createBackup", None,
+                           backup_type=backup_type,
+                           rotation=rotation,
+                           name=name)
+
     def change_password(self, server_id, password):
         return self.action(server_id, "changePassword", None,
                            adminPass=password)