document manual_cache_clean better

[Scott Moser]

LP: #1885527 raised (not for the first time) a general failure of cloud-init's
documentation to cover 'manual_cache_clean'. In fact, this configuration
value not referenced at all in readthedocs, but only in
doc/examples/cloud-config.txt
  https://github.com/canonical/cloud-init/blob/b70aec8e5ed59298e9fbd5da449350dd3d0002d2/doc/examples/cloud-config.txt#L467

The intent (testing is needed) for manual_cache_clean is:

a.) user-data and system config (/etc/cloud/*.cfg) can set
manual_cache_clean to true or false. As always, user-data overrides system
config. vendor-data should also be able to provide the setting.

b.) cloud-init renders /var/lib/cloud/instance/manual-clean
(path_helper.get_ipath_cur("manual_clean_marker"))

c.) on boot, both ds-identify and cloud-init will check
and respect existance of /var/lib/cloud/instance/manual-clean .
If that file is present, then cloud-init will not make any
attempts to re-discover a metadata service.

So... "unfreeze", if manual_cache_clean was set is just:
 rm -Rf /var/lib/cloud/instance /var/lib/cloud/instance/

I think it would be good to both test that my intent/understanding are
correct, and document it. Also useful might be documenting use case
that makes this necessary which is described in:
  https://bugs.launchpad.net/ubuntu/+source/cloud-init/+bug/1885527/comments/6

Related bugs:
 * bug 1885527: cloud-init regenerating ssh-keys
 * bug 1712680: cloud-init re-generates network config every reboot

[Scott Moser (smoser)]

https://github.com/canonical/cloud-init/pull/568

[Launchpad Janitor (janitor)]

This bug was fixed in the package cloud-init - 20.4-0ubuntu1

---------------
cloud-init (20.4-0ubuntu1) hirsute; urgency=medium

  * d/control: add gnupg to Recommends as cc_apt_configure requires it to be
    installed for some operations.
  * New upstream release.
    - Release 20.4 (#686) [James Falcon] (LP: #1905440)
    - tox: avoid tox testenv subsvars for xenial support (#684)
    - Ensure proper root permissions in integration tests (#664) [James Falcon]
    - LXD VM support in integration tests (#678) [James Falcon]
    - Integration test for fallocate falling back to dd (#681) [James Falcon]
    - .travis.yml: correctly integration test the built .deb (#683)
    - Ability to hot-attach NICs to preprovisioned VMs before reprovisioning
      (#613) [aswinrajamannar]
    - Support configuring SSH host certificates. (#660) [Jonathan Lung]
    - add integration test for LP: #1900837 (#679)
    - cc_resizefs on FreeBSD: Fix _can_skip_ufs_resize (#655)
      [Mina Galić] (LP: #1901958, #1901958)
    - DataSourceAzure: push dmesg log to KVP (#670) [Anh Vo]
    - Make mount in place for tests work (#667) [James Falcon]
    - integration_tests: restore emission of settings to log (#657)
    - DataSourceAzure: update password for defuser if exists (#671) [Anh Vo]
    - tox.ini: only select "ci" marked tests for CI runs (#677)
    - Azure helper: Increase Azure Endpoint HTTP retries (#619) [Johnson Shi]
    - DataSourceAzure: send failure signal on Azure datasource failure (#594)
      [Johnson Shi]
    - test_persistence: simplify VersionIsPoppedFromState (#674)
    - only run a subset of integration tests in CI (#672)
    - cli: add --system param to allow validating system user-data on a
      machine (#575)
    - test_persistence: add VersionIsPoppedFromState test (#673)
    - introduce an upgrade framework and related testing (#659)
    - add --no-tty option to gpg (#669) [Till Riedel] (LP: #1813396)
    - Pin pycloudlib to a working commit (#666) [James Falcon]
    - DataSourceOpenNebula: exclude SRANDOM from context output (#665)
    - cloud_tests: add hirsute release definition (#662)
    - split integration and cloud_tests requirements (#652)
    - faq.rst: add warning to answer that suggests running `clean` (#661)
    - Fix stacktrace in DataSourceRbxCloud if no metadata disk is found (#632)
      [Scott Moser]
    - Make wakeonlan Network Config v2 setting actually work (#626)
      [dermotbradley]
    - HACKING.md: unify network-refactoring namespace (#658) [Mina Galić]
    - replace usage of dmidecode with kenv on FreeBSD (#621) [Mina Galić]
    - Prevent timeout on travis integration tests. (#651) [James Falcon]
    - azure: enable pushing the log to KVP from the last pushed byte (#614)
      [Moustafa Moustafa]
    - Fix launch_kwargs bug in integration tests (#654) [James Falcon]
    - split read_fs_info into linux & freebsd parts (#625) [Mina Galić]
    - PULL_REQUEST_TEMPLATE.md: expand commit message section (#642)
    - Make some language improvements in growpart documentation (#649)
      [Shane Frasier]
    - Revert ".travis.yml: use a known-working version of lxd (#643)" (#650)
    - Fix not sourcing default 50-cloud-in...