default_min_hwe_kernel: hwe-w is too early to be applied

ppc64el env I'm currently using requires hwe-w for commissioning. With maas-deployer capability to configure maas settings, I wrote the yaml like below.

====
        settings:
            main_archive: http://archive.ubuntu.com/ubuntu
            maas_name: demo
            default_min_hwe_kernel: hwe-w
====

However setting default_min_hwe_kernel always fails.

====
2016-04-13 17:47:55,052 DEBUG (execc) Executing: 'ssh -i /home/ubuntu/.ssh/id_maas -o UserKnownHostsFile=/dev/null -o StrictHostKeyChecking=no -o LogLevel=quiet ubuntu@172.X.Y.Z maas maas maas set-config name='default_min_hwe_kernel' value='hwe-w'' stdin='LC_ALL=C'
2016-04-13 17:47:55,582 ERROR (execc)
2016-04-13 17:47:55,583 ERROR (_maas_execute) Command 'ssh -i /home/ubuntu/.ssh/id_maas -o UserKnownHostsFile=/dev/null -o StrictHostKeyChecking=no -o LogLevel=quiet ubuntu@172.X.Y.X maas maas maas set-config name='default_min_hwe_kernel' value='hwe-w'' failed: rc='2' output=''
2016-04-13 17:47:55,583 ERROR (apply_maas_settings) Unable to set default_min_hwe_kernel to hwe-w
====

It seems default_min_hwe_kernel cannot be set before importing boot images. On the other hand, http_proxy in maas config is required before importing boot images...

[maas_deployer/vmaas/engine.py]
         self.apply_maas_settings(client, maas_config)
         self.wait_for_import_boot_images(client, maas_config)
         self.configure_maas(client, maas_config)