Fix: partitions recently imaged not being detected

When we have just written out a whole disk image, if we do not run
partprobe none of the new partitions show up in blkid, therefore even
images with a built-in configdrive partition will have one created

Change-Id: I6ec98dc237b42bfe94ce79cf84d155332335c9f6
Merge "Fix: partitions recently imaged not being detected"