Place root partition last so that it can always be expanded

Make the partitioning layout consistent by having the root partition
always be the last partition. This enables tools like cloud-init's
growroot utility to expand the root partition until the end of the disk,
regardless of the presence of a swap or ephemeral partition. In the
case where there is neither a swap nor ephemeral partition, the root
partition is the only partition created.

Closes-Bug: #1313606
Change-Id: I82492eb50530adc1f31e934f15f5c67462819a71
Merge "Place root partition last so that it can always be expanded"