Allow configdrive partition to be precreated

This adds support for copying configdrives to disks that already have
partitions enabled and labeled for configdrives. This is neccessary for
supporting configdrives on partitioning schemes we don't support yet
(like GPT) or for some future agent iteration where we create partitions
separately.

Change-Id: Iee5b87d82be08febc495aaef3272ced4f2f32235
Merge "Allow configdrive partition to be precreated"