Two nodes commissioning failed with the following error.

 [ERROR] node01: Marking node failed: Node operation 'Commissioning' timed out after 0:20:00.

maas.log
========
Sep 14 07:39:07 maas-server maas.dns: [INFO] Generating new DNS zone file for maas
Sep 14 07:39:07 maas-server maas.dns: [INFO] Generating new DNS zone file for 122.168.192.in-addr.arpa
Sep 14 07:39:08 maas-server maas.drivers.virsh: [INFO] node04: Successfully set network boot order
Sep 14 07:39:20 maas-server maas.power: [INFO] node04: Power state has changed from unknown to off.
Sep 14 07:42:23 maas-server maas.node: [INFO] node01: Status transition from NEW to COMMISSIONING
Sep 14 07:42:23 maas-server maas.node: [INFO] node04: Status transition from NEW to COMMISSIONING
Sep 14 07:42:23 maas-server maas.node: [INFO] node02: Status transition from NEW to COMMISSIONING
Sep 14 07:42:23 maas-server maas.node: [INFO] node03: Status transition from NEW to COMMISSIONING
Sep 14 07:42:23 maas-server maas.power: [INFO] Changing power state (on) of node: node01 (node-97f01944-7a80-11e6-af8e-5254000f6110)
Sep 14 07:42:23 maas-server maas.node: [INFO] node01: Commissioning started
Sep 14 07:42:23 maas-server maas.node: [INFO] node02: Commissioning started
Sep 14 07:42:23 maas-server maas.node: [INFO] node04: Commissioning started
Sep 14 07:42:23 maas-server maas.node: [INFO] node03: Commissioning started
Sep 14 07:42:23 maas-server maas.power: [INFO] Changing power state (on) of node: node02 (node-991df5b6-7a80-11e6-98fd-5254000f6110)
Sep 14 07:42:23 maas-server maas.power: [INFO] Changing power state (on) of node: node04 (node-9ba86f8c-7a80-11e6-a907-5254000f6110)
Sep 14 07:42:23 maas-server maas.power: [INFO] Changing power state (on) of node: node03 (node-9a49bb96-7a80-11e6-9a02-5254000f6110)
Sep 14 07:42:28 maas-server maas.power: [INFO] Changed power state (on) of node: node01 (node-97f01944-7a80-11e6-af8e-5254000f6110)
Sep 14 07:42:28 maas-server maas.power: [INFO] Changed power state (on) of node: node03 (node-9a49bb96-7a80-11e6-9a02-5254000f6110)
Sep 14 07:42:28 maas-server maas.power: [INFO] Changed power state (on) of node: node02 (node-991df5b6-7a80-11e6-98fd-5254000f6110)
Sep 14 07:42:29 maas-server maas.power: [INFO] Changed power state (on) of node: node04 (node-9ba86f8c-7a80-11e6-a907-5254000f6110)
Sep 14 07:44:05 maas-server maas.lease_upload_service: [INFO] Uploading 26 DHCP leases to region controller.
Sep 14 07:45:05 maas-server maas.lease_upload_service: [INFO] Uploading 28 DHCP leases to region controller.
Sep 14 07:45:08 maas-server maas.node: [INFO] node03: Status transition from COMMISSIONING to READY
Sep 14 07:45:10 maas-server maas.node: [INFO] node02: Status transition from COMMISSIONING to READY
Sep 14 07:45:29 maas-server maas.node_query: [INFO] node03: Power is on.
Sep 14 07:45:30 maas-server maas.node_query: [INFO] node02: Power is on.
Sep 14 07:46:05 maas-server maas.lease_upload_service: [INFO] Uploading 35 DHCP leases to region controller.
Sep 14 07:55:04 maas-server maas.import-images: [INFO] Started importing boot images.
Sep 14 07:55:05 maas-server maas.import-images: [INFO] Finished importing boot images, the region does not have any new images.
Sep 14 08:02:23 maas-server maas.node: [INFO] node04: Status transition from COMMISSIONING to FAILED_COMMISSIONING
Sep 14 08:02:23 maas-server maas.node: [ERROR] node04: Marking node failed: Node operation 'Commissioning' timed out after 0:20:00.
Sep 14 08:02:23 maas-server maas.node: [INFO] node01: Status transition from COMMISSIONING to FAILED_COMMISSIONING
Sep 14 08:02:23 maas-server maas.node: [ERROR] node01: Marking node failed: Node operation 'Commissioning' timed out after 0:20:00.
Sep 14 08:02:43 maas-server maas.node_query: [INFO] node01: Power is on.
Sep 14 08:02:43 maas-server maas.node_query: [INFO] node04: Power is on.