Use dib-run-parts on dib-first-boot.

At least on RedHat the run-parts command is very precarious and doesn't
even check for the return code of the scripts it runs causing the
dib-first-boot to delete itself even when something went wrong with the
configuration of the first boot of the image. Using the dib-run-parts
instead will make the whole first boot routine to be more robust and
less platform dependent.

Change-Id: I0073fd52fc1fc8848ca9277a84603124215cdb6e
Merge "Use dib-run-parts on dib-first-boot."