Enable custom apt.conf in apt-conf element

Allow DIB_APT_CONF to be set to the apt.conf file to be inserted
into the built images.

Change-Id: I5d148ae7286a93e81b3e984c94ba4f089d3a3280

elements/apt-conf/README.md

@@ -0,0 +1,18 @@
+Override the default apt.conf
+=============================
+
+For APT based systems, if you require specific options for apt operations,
+you can define `DIB_APT_CONF` with your favorite apt.conf to override it,
+before running devtest.sh.
+
+The new apt.conf will take effect at build time and run time.
+
+If you want to use this element with tripleo-incubator scripts, set `NODE_DIST`
+/ `SEED_DIB_EXTRA_ARGS` / `UNDERCLOUD_DIB_EXTRA_ARGS` /
+`OVERCLOUD_CONTROL_DIB_EXTRA_ARGS` / `OVERCLOUD_COMPUTE_DIB_EXTRA_ARGS` to
+make it take effect at the appropriate time.
+
+For instance, before running devtest.sh:
+
+    export DIB_APT_CONF=/etc/apt/apt.conf
+    export NODE_DIST="ubuntu apt-conf"

elements/apt-conf/extra-data.d/99-override-default-apt-conf

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+#!/bin/bash
+# Override the default /etc/apt/apt.conf with $DIB_APT_CONF
+
+set -eu
+
+# exit directly if DIB_APT_CONF is not defined properly
+if [ -z "$DIB_APT_CONF" ] ; then
+    echo "DIB_APT_CONF is not set - no apt.conf will be copied in"
+    exit 0
+elif [ ! -f "$DIB_APT_CONF" ] ; then
+    echo "$DIB_APT_CONF is not a valid apt.conf file."
+    echo "You should assign a proper apt.conf file in DIB_APT_CONF"
+    exit 1
+fi
+
+# copy the apt.conf to cloudimg
+sudo cp -L -f $DIB_APT_CONF $TMP_MOUNT_PATH/etc/apt/apt.conf