setting manual_cache_clean causes warning

[Scott Moser]

If manual_cache_clean is set in an instance, then ds-identify will write
 '# manual_cache_clean."'
to its config (/run/cloud-init/cloud.cfg).

cloud-init will try to load that file, and will WARN on it not being a dict.

2018-01-10 15:45:57,282 - util.py[WARNING]: Failed loading yaml blob
2018-01-10 15:45:57,283 - util.py[DEBUG]: Failed loading yaml blob
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/cloudinit/util.py", line 898, in load_yaml
    (allowed, type_utils.obj_name(converted)))
TypeError: Yaml load allows (<class 'dict'>,) root types, but got NoneType instead

python3 -c 'import yaml; yaml.load("# manual_cache_clean.\n")'
 echo "manual_cache_clean: true" > /etc/cloud/cloud.cfg.d/manual.cfg
If you set manual_cache_clean in an instance, then

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: cloud-init 17.2-1-g4089e20c-0ubuntu1
ProcVersionSignature: Ubuntu 4.13.0-17.20-generic 4.13.8
Uname: Linux 4.13.0-17-generic x86_64
ApportVersion: 2.20.8-0ubuntu5
Architecture: amd64
CloudName: NoCloud
Date: Wed Jan 10 15:46:20 2018
PackageArchitecture: all
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 LANG=C.UTF-8
SourcePackage: cloud-init
UpgradeStatus: No upgrade log present (probably fresh install)
user_data.txt:
 #cloud-config
 {}

[Chad Smith (chad.smith)]

This is done in ./src/maasserver/compose_preseed.py

        # Prevent the node from requesting cloud-init data on every reboot.
        # This is done so a machine does not need to contact MAAS every time
        # it reboots.
        "manual_cache_clean": True,

[Launchpad Janitor (janitor)]

This bug was fixed in the package cloud-init - 17.2-13-g6299e8d0-0ubuntu1

---------------
cloud-init (17.2-13-g6299e8d0-0ubuntu1) bionic; urgency=medium

  * New upstream snapshot.
    - Do not log warning on config files that represent None. (LP: #1742479)
    - tests: Use git hash pip dependency format for pylxd. [Chad Smith]
    - tests: add integration requirements text file [Joshua Powers]
    - MAAS: add check_instance_id based off oauth tokens. (LP: #1712680)

 -- Scott Moser <email address hidden> Mon, 15 Jan 2018 10:15:05 -0500

[Scott Moser (smoser)]

This bug is believed to be fixed in cloud-init in 18.1. If this is still a problem for you, please make a comment and set the state back to New

Thank you.

[James Falcon (falcojr)]

Tracked in Github Issues as https://github.com/canonical/cloud-init/issues/3094