MAAS/Curtin: Allow automatic calculation of remaining partition space when configuring disks in curtin

In MAAS, when configuring curtin preseeds, allow the parsing of “-1” as a indicator to use “remaining free space” in the partitioning sections.

So for example instead of inputting the exact extent of the partition, the feature request is for curtin (and therefore MAAS) to be able to accept “-1” or some other symbol for “size:” so that the remaining partition space is automatically calculated/used without the user having to explicit enter a size:

storage:
  config:
    id: part-0
    device: disk-0
    size: 1048576
    flag: bios_grub
  - type: partition
    id: part-1
    device: disk-0
    size: -1

Such a mechanism is already in place for LVM partitions as documented in the curtin docs (
Storage — curtin 23.1.1 documentation)

So the ask here is to replicate the same behavior for generic partition layouts

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This should be simple enough to do… and seems like really low-hanging fruit that would save hassle in needing to manually calculate end of disk for sgdisk to use…