Replace part_handler.py with write-files cloudinit mechanism

part-handler.py was acting as a write-files mechanism.  Instead just
use the write-files mechanism directly to avoid the complexities of
the part-handler.

blueprint: use-cloudinit-write
Change-Id: I6e80c344743d6fd2fa9a49507de6d50e3d9eea73
Merge "Replace part_handler.py with write-files cloudinit mechanism"