[2.3+] Unable to disk erase if machine is deployed with a non-lts kernel

I have asked maas 2.2.2 to erase disks as they are released. This works great for xenial but fails on machines deployed with artful. Here is what happens.

1) In maasserver/models/node.py the release_or_erase function calls either the start_disk_erasing or release function depending on whether the disks need to be erased or not. Note: release clears the distro and hwe_kernel flags from the node object but start_disk_erasing does not.
2) The machine is rebooted and requests its grub.cfg which results in a function call to get_config in maasserver/rpc/boot.py. From there, we get the purpose of the boot by calling function get_boot_purpose defined in maasserver/rpc/boot.py. The boot purpose returned is "commissioning" because "The environment (boot images, kernel options, etc for erasing is the same as that of commissioning." (as documented in the comments)
3) Since the boot purpose is commissioning we decide to overwrite the system and series but we do not modify the architecture or hwe_kernel.
4) Still in get_config we validate this combination of artful kernel with a xenial series and promptly throw the following error and give up on erasing the disk.
maas.node: [error] hostname: Marking node failed: Missing boot image ubuntu/amd64/ga-17.10/xenial.

My hack/workaround is as follows in case you are interested.

diff --git a/maasserver/rpc/boot.py b/maasserver/rpc/boot.py
index 5ec41bb..83d58d3 100644
--- a/maasserver/rpc/boot.py
+++ b/maasserver/rpc/boot.py
@@ -199,6 +199,9 @@ def get_config(
         if purpose == "commissioning":
             osystem = Config.objects.get_config('commissioning_osystem')
             series = Config.objects.get_config('commissioning_distro_series')
+ subarch = "generic"
+ machine.architecture = '{}/{}'.format(arch, subarch)
+ machine.hwe_kernel = None
         else:
             osystem = machine.get_osystem()
             series = machine.get_distro_series()