Machine-specific minimal commissioning kernel resets to MAAS wide default

[Andrew Cloke]

When trying to commission a new node that requires a min of bionic-hwe to commission, I:
* go to "Machine Configuration" page
* select "Minimum Kernel" to be bionic-hwe
* select "Save Changes"
* go to "Take action" -> "Commission"
* Minimum kernel then jumps back to bionic-ga
* And commissioning fails

However, when I set the MAAS-wide minimum commissioning kernel to bionic-hwe, it sticks and I can successfully commission the machine.

I can then go back and reset the MAAS-wide minimum commissioning kernel to bionic, as that's most applicable to the majority of the nodes controlled by this MAAS.

[Alberto Donato (ack)]

This is similar to https://bugs.launchpad.net/maas/+bug/1811358.

The current logic (see https://bugs.launchpad.net/maas/+bug/1811358/comments/5 specifically) is to unconditionally set the min_hwe_kernel for a machine to the default one in Node.start_commissioning.
I think we should not modify the setting, but rather pick the machine-specific one or fall back to the default one.

[Brett Milford (brettmilford)]

We have a customer with this issue.
They need to override kernel version on a per machine basis, for newer machines which require the latest kernel.