Revert "Replace part_handler.py with write-files cloudinit mechanism"

Ubuntu has 0.6 of cloudinit, and write-files doesn't work on that
distro.  Ubuntu does not intend to update cloudinit in their LTS release
to 0.7.

This reverts commit 621f5bfdbab46498b37cdc4865afa57e0f0498e0.
Fixes: Bug #1207088

Change-Id: If80863883afee28bdde6dd506826ec5710cc0308
Merge "Revert "Replace part_handler.py with write-files cloudinit mechanism""