Add default value "3m" to image_delay, making it consistent with docs.

[zhiguangwang]

Currently, image_delay defaults to 0, causing nomad client to delete docker images the moment they become unused.

This is not the expected behavior according to docs:

https://www.nomadproject.io/docs/drivers/docker.html#image_delay

[hashicorp-cla]

All committers have signed the CLA.

[langmartin]

Thanks for this! It does match the documentation. I'm going to ask for one more confirmation, but it looks good to me.

[notnoop]

Thanks - I think i've been hit by nomad deleting images prematurely without fully understanding why. Thanks for catching this.

[langmartin]

I think this change just needs to be rebased on master in order for the tests to pass

[zhiguangwang]

I think this change just needs to be rebased on master in order for the tests to pass

Done :)

[endocrimes]

Thanks - This was actually catching me a bunch last week but I didn't get a chance to debug

[github-actions]

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