Add new cloud-init element

Allows for composable builds from a minimal source.

Change-Id: I16f560ca4b1737c0928f3f3b15ea2580df820e97
Closes-Bug: 1539272

elements/cloud-init/README.rst

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+========
+cloud-init
+========
+
+Install's and enables cloud-init for systems that don't come with it
+pre-installed
+
+Currently only supports Gentoo.

elements/cloud-init/element-deps

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+package-installs

elements/cloud-init/package-installs.yaml

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+cloud-init:

elements/cloud-init/pkg-map

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+{
+  "family": {
+    "gentoo": {
+      "cloud-init": "app-emulation/cloud-init"
+    }
+  }
+}

elements/cloud-init/post-install.d/20-enable-cloud-init

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+#!/bin/bash
+
+if [ ${DIB_DEBUG_TRACE:-0} -gt 0 ]; then
+    set -x
+fi
+set -eu
+set -o pipefail
+
+# gentoo
+if [[ "${DISTRO_NAME}" == "gentoo" ]]; then
+    rc-update add cloud-config default
+    rc-update add cloud-final default
+    rc-update add cloud-init-local default
+    rc-update add cloud-init default
+fi