Comment 8 for bug 1835087

I'm in agreement on the curtin fix there; however, I do think it wise in subiquity to make sure that it's not setting ptable when it wasn't if preserve is set to true.

A quick look shows that the 'add_partition()' method will set a disk.ptable attribute to 'gpt'. I've not walked through all of the flows to be certain that it won't get set.