Implement exec on Directories

[sbrow]

As discussed in issue #769. This is still a work in progress.

Todos:

• Write tests
• Implement in plugins other than docker.
• Add option to enable in volume.NewFS?
• Update entrySchema on execDir.

[welcome]

Hi! 💖 Thanks for your contribution 🎉🥳

We'll get it reviewed as soon as possible. If you'd like to chat with the developers, the #puppet-dev channel on Slack would probably be a great place to start. Make sure that your commit messages fit our contribution standards to make the code review go quickly.

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[MikaelSmith]

That looks pretty straightforward, although it looks like some test mocking needs to be tweaked.

[CLAassistant]

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