MAAS doesn't allow you to select what HWE kernel to use when commissioning

[Andres Rodriguez]

MAAS doesn't allow you to select the HWE kernel to use for commissioning. It, however, allows you to change the MAAS release. It would be nice if MAAS would allow you to change the commissioning release *and* the HWE kernel in use.

1. Global option (global_min_hwe_kernel = hwe-v)
2. Machine enlists using global_min_hwe_kernel
3. after enlistment, machine's min_hwe_kernel is set = to min_hwe_kernel_ephemeral (node.min_hwe_kernel = global_min_hwe_kernel)
4. commissioning uses node.min_hwe_kernel if set (or generic if none set)
5. deployment uses min_hwe_kernel if set (which is what currently happens)

[Mark Shuttleworth (sabdfl)]

Would it be possible to know that a machine FETCHED a kernel but then FAILED to boot it properly? If so, could we not automagically:

 * try the default release kernel, and if needed
 * try the next kernel up, and if needed
 * keep going through the HWE kernels, then
 * record the first one that works?

In this way, we could automagically "just work" when commissioning, automagically remember the needed HWE kernel, but still let someone set the kernel to use as a property of the machine if they want to do it manually?