Add a pip-cache element.

The element bind mounts a pip cache inside the image build chroot so
that pip downloads can be reused across image builds.  While similar
in purpose to the PyPi element that sets up a mirror, this element
just allows for a reusable download cache and doesn't require anything
to be setup beforehand.

The pip-cache element is not concurrency safe, and that is indicated
in the README for the element.  An upstream bug was file as well:
https://github.com/pypa/pip/issues/1141

Change-Id: Ibd1d4ea17c24923ed939357ada95b781e3179cfd

elements/pip-cache/README.md

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+# Use a cache for pip
+
+Using a download cache speeds up image builds.
+
+Including this element in an image build causes
+$HOME/.cache/image-create/pip to be bind mounted as /tmp/pip inside
+the image build chroot.  The $PIP_DOWNLOAD_CACHE environment variable
+is then defined as /tmp/pip, which causes pip to cache all downloads
+to the defined location.
+
+Note that pip and its use of $PIP_DOWNLOAD_CACHE is not concurrency
+safe.  Running multiple instances of diskimage-builder concurrently
+can cause issues.  Therefore, it is advised to only have one instance
+of diskimage-builder that includes the pip-cache element running at a
+time.
+
+The pip concurrency issue is being tracked upstream at
+https://github.com/pypa/pip/issues/1141

elements/pip-cache/environment.d/10-pip-cache

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+export PIP_DOWNLOAD_CACHE=/tmp/pip

elements/pip-cache/root.d/01-pip-cache

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+#!/bin/bash
+
+set -e
+
+PIP_CACHE_DIR=$HOME/.cache/image-create/pip
+mkdir -p $PIP_CACHE_DIR
+
+sudo mkdir -p $TMP_MOUNT_PATH/tmp/pip
+sudo mount --bind $PIP_CACHE_DIR $TMP_MOUNT_PATH/tmp/pip