chore(cmd-modules): deprecate --mode init

[aciba90]

Proposed Commit Message

chore(cmd-modules): deprecate --mode init

This code path is not excuted by cloud-init at boot time and can
potentially mislead people.

Deprecate `cloud-init modules --mode init` and point to
`cloud-init init` as the recommended cmd.

Additional Context

Test Steps

cloud-init modules -m init
cloud-init modules -m init --help

Checklist

• My code follows the process laid out in the documentation
• I have updated or added any unit tests accordingly
• I have updated or added any documentation accordingly

Merge type

• Squash merge using "Proposed Commit Message"
• Rebase and merge unique commits. Requires commit messages per-commit each referencing the pull request number (#<PR_NUM>)

[holmanb]

nit: Think we should add something to the cli --help output as well? Also I think I recall this being documented somewhere, maybe on the cli docs page.

+1

[aciba90]

nit: Think we should add something to the cli --help output as well? Also I think I recall this being documented somewhere, maybe on the cli docs page.

+1

Good point, I have included deprecation messages in the cmd --help and rtd, thanks!