Allow interpolation of build variables in http_content

[dbnicholson]

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Description

Currently you can't interpolate build variables such as {{ .HTTPIP }} in http_content values. If you try to use one, the rendered value is no value.

Use Case(s)

I'd like to use {{ .SSHPublicKey }} in http_content so that it can be used in cloud-init metadata, but I can think of any number of why you'd want to interpolate build variables directly into HTTP content. The background is I'm trying to use a Debian cloud image locally to mock the AWS image we normally use. In those images, cloud-init is the only entrypoint.

Potential configuration

http_content = {
  "/pubkey" = "{{ .SSHPublicKey }}"
}

Potential References

#109