Match vFAT label using a given label instead of hardcoding 'config-2'.
The NoCloud and ConfigDrive metadata services both attempt to find vFAT
formatted drives but the label used was hardcoded to 'config-2' which
only works with ConfigDrive. BaseConfigDriveService which both inherit
from already has a drive_label property set appropriately for both so
let's use that for the vFAT finding logic as well.
Change-Id: I8004a8565338b0615450bb28cecc86901be94766
9b17eaf1d4