Switch server backup to sdk.

Switch this command from novaclient to SDK. As this is the first command
related to server that we are migrating, we need to extend our test
fakes to support fake Server resources. The extended fakes will replace
the old ones once all commands related to server are switched.

Change-Id: If476fb1614a64320ed071bbda35e941bf3290a2e

Author: diweizhu

openstackclient/compute/v2/server_backup.py

@@ -72,12 +72,9 @@ def _show_progress(progress):
                 self.app.stderr.write('\rProgress: %s' % progress)
                 self.app.stderr.flush()
 
-        compute_client = self.app.client_manager.compute
+        compute_client = self.app.client_manager.sdk_connection.compute
 
-        server = utils.find_resource(
-            compute_client.servers,
-            parsed_args.server,
-        )
+        server = compute_client.find_server(parsed_args.server)
 
         # Set sane defaults as this API wants all mouths to be fed
         if parsed_args.name is None:
@@ -93,7 +90,7 @@ def _show_progress(progress):
         else:
             backup_rotation = parsed_args.rotate
 
-        compute_client.servers.backup(
+        compute_client.backup_server(
             server.id,
             backup_name,
             backup_type,