Curtin fails to assemble software RAID members

=== Description ===

Sometimes (50%) when I deploy nodes in MaaS with Ubuntu Trusty and "hwe-x" kernel software RAID doesn't get configured and the installation fails with the following error:

Error occurred handling 'sdh': ProcessExecutionError - Unexpected error while running command.
Command: ['mdadm', '--assemble', '--scan']
Exit code: 3
Rason: -
Stdout: ''
Stderr: u'mdadm: /dev/md/4 assembled from 1 drive - not enough to start the array.\n'
Uexpected error while running command.
Command: ['mdadm', '--assemble', '--scan']
Eit code: 3
Reason: -
Stdout: ''
Stderr: u'mdadm: /dev/md/4 assembled from 1 drive - not enough to start the array.\n'
Installation failed with exception: Unexpected error while running command.
Command: ['curtin', 'block-meta', 'custom']
Exit code: 3
Reason: -

Usually, it is sufficient to release and re-deploy the node. I cannot use hardware RAID, because disks are not plugged into a RAID controller.

=== Additional information ===

1. Contents of /var/log/maas/*:

Attached (logs.tar.gz)

2. Output from dpkg -l '*maas*'|cat

root@optionsit-os-maas-1:~# dpkg -l '*maas*'|cat
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name Version Architecture Description
+++-==================================-=================================-============-===============================================================================
ii maas 1.9.4+bzr4592-0ubuntu1~trusty1 all MAAS server all-in-one metapackage
ii maas-cli 1.9.4+bzr4592-0ubuntu1~trusty1 all MAAS command line API tool
ii maas-cluster-controller 1.9.4+bzr4592-0ubuntu1~trusty1 all MAAS server cluster controller
ii maas-common 1.9.4+bzr4592-0ubuntu1~trusty1 all MAAS server common files
ii maas-dhcp 1.9.4+bzr4592-0ubuntu1~trusty1 all MAAS DHCP server
ii maas-dns 1.9.4+bzr4592-0ubuntu1~trusty1 all MAAS DNS server
ii maas-proxy 1.9.4+bzr4592-0ubuntu1~trusty1 all MAAS Caching Proxy
ii maas-region-controller 1.9.4+bzr4592-0ubuntu1~trusty1 all MAAS server complete region controller
ii maas-region-controller-min 1.9.4+bzr4592-0ubuntu1~trusty1 all MAAS Server minimum region controller
ii python-django-maas 1.9.4+bzr4592-0ubuntu1~trusty1 all MAAS server Django web framework
ii python-maas-client 1.9.4+bzr4592-0ubuntu1~trusty1 all MAAS python API client
ii python-maas-provisioningserver 1.9.4+bzr4592-0ubuntu1~trusty1 all MAAS server provisioning libraries

3. Instructions on how we can re-create your bug and what the expected result should be.

* Install MaaS according to the version provided above
* Enlist and commission a node with at least two disks
* Configure software RAID from MaaS
* Deploy the node with Ubuntu Trusty ad "hwe-x" kernel