Respect the preset permissions of config files

It's sometimes desirable to specify the permissions or ownership of the
generated config file instead of setting the default 644 root:root.

If the file is already present, os-apply-config will just update the
contents and leave everything else alone.

Closes-Bug: #1246266
Change-Id: Idd41ec276dd04f517760c1dbd97c75e376840b41