add "boot" provisioning script mode

[pendo324]

Allows users to customize their early boot workflow by specifying bootCmds. These commands are run by cloud-init-local.service, which runs before cloud-init.service (which only starts after the network is up), and allow users to disable services (as in the example) or run other commands early on in the boot process. More info here.

Resolves one of the proposals in #1093

[AkihiroSuda]

Thanks, but please sign the commit for DCO
https://github.com/apps/dco

(run git commit -a -s --amend, and make sure that the Signed-off-by: NAME <EMAIL> line with your real name is included in the commit message)

[AkihiroSuda]

Thanks

[AkihiroSuda]

Could you squash commits?

[jandubois]

Thank you for the contribution! I've tested it, and it works well.

I would like to see the misleading hash-bang line issue addressed.

I wouldn't insist on fixing the YAML indentation, but if you edit the PR anyways, then please address that as well.

[pendo324]

Thanks for all the feedback. I just made all of the changes that were requested regarding the misleading example and the YAML formatting, and also rebased on latest master and squashed into one commit

[jandubois]

Thank you, looks good!

[jandubois]

I don't know why the Cirrus CI checks have not been running; I'm going to close this PR and then re-open it, to hopefully trigger them again.

[jandubois]

Doing another cycle of Close+Reopen, as Cirrus-CI seems to be working fine in other repos right now; but still not triggering here.