Make os-apply-config read on-disk config files

os-collect-config was changed recently to save the list of collected
files on disk, and not just in OS_CONFIG_FILES. This will allow
os-apply-config to be run outside of os-collect-config and still have
access to the latest copies of collected metadata.

Change-Id: I520f1332e5789e24562300f38af02b37a062ab48