Add AfterFiles dependencies to File tasks

[louismunro]

This adds a new "AfterFiles" field to the File task.
It allows File tasks to depend on the creation of another file first.
The typical use case would be for a service which depends on multiple files and must not be restarted before they are all written to disk, such as Docker.

Fixes #4747

(I am sure this can be improved. I am putting it out there as a WIP and a basis for discussion of #4747).
Feel free to squash the commits.

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[robinpercy]

Thanks @louismunro! Please sign the CLA when you get a minute.

/assign

[justinsb]

This is really great - looks totally perfect and ready to go (but squashing the commits would be good) - apart from the CLA :-( Are you able to sign the CLA - it is a bit of a pain, but then it is the same CLA across all the k8s projects.

[justinsb]

/ok-to-test

[louismunro]

I just signed the CLA.

[robinpercy]

/retest

[louismunro]

I think the only thing that's holding it now is that I have a commit authored with another email address than the one associated with my github account. Is that a problem?
I can squash the commits myself or let whoever merges do it. Just let me know.

[louismunro]

I squashed all my commits. I believe that should take care of the CLA issue.

[KashifSaadat]

@louismunro perfect, thanks for the fix and for signing the CLA. :)

/lgtm

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: KashifSaadat, louismunro

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