storage: fix use of ptable parameter when calling reformat

[ogayot]

When reformatting a disk using the following call:

manipulator.reformat(disk, ptable="something")

The ptable parameter would only appear to be honored if the disk did not contain any partition. This happened because the reformat() function sets the ptable of the disk first and then starts removing partitions using delete_partition() in a loop.

However, delete_partition() has a mechanism to reset the partition table to None when it is invoked on the last partition of a disk.

Therefore, the disk ended up with None as the partition table (ignoring the ptable value) if the disk contained any partition.

Fixed by moving the assignment of disk.ptable after removing partitions. Arguably, it is a mistake to change the partition table value before removing the partitions anyway.

[ogayot]

Updated to change the meaning of reformat(disk, ptable=None)/

Previously this meant "keep the current type of partition table" but it was only honored when the disk had zero partition. The new meaning is "use the default partition type", which is what we have always, accidentally, done for disks with partitions.

The reason for introducing this potentially breaking change is that formatting a disk that had partitions is a more common use-case than formatting a disk that has zero partition. At this point, we sort of expect reformat(disk, ptable=None) to create a default/GPT partition table so "fixing" the original issue will be more disruptive than changing the meaning of reformat(ptable=None).

I introduced the ability in autoinstall to specify the type of partition table when doing reformat, so that if anyone wanted to keep the current partition table format, they can still do it (provided they know what the previous type was).