Use full path to dib-run-parts during firstboot.

Commit cc5ecb4e9cb29c45369cc07014c15202f4a5fd78 switched from using
run-parts in favor of dib-run-parts.

/usr/local/bin isn't part of PATH on Ubuntu causing
"/usr/sbin/dib-first-boot: line 8: dib-run-parts: command not found".

Change-Id: Ia7e401221add197edf880af82e0148f3ba7d2b9c