Adds check mode support for docker_puppet. The updated json file is
written to /var/lib/docker-puppet/check-mode/docker-puppet.json
during check mode and then diffed with the existing version at
/var/lib/docker-puppet/docker-puppet.json.
When docker-puppet.py is run during check mode, the updated json file
under the check-mode directory is passed to the command. All generated
config files are then written under /var/lib/config-data/check-mode,
which is then recursively diffed with the existing config under just
/var/lib/config-data to report on all changed config files.
Change-Id: I5c831e9546f8b6edaf3b0fda6c9fbef86c825a4c
a6f9821821