Add pip-and-virtualenv element

This element allows installation of pip and virtualenv from either
distro packages or git.

Change-Id: Id294f0936c8fef8a3b27a415bfcc93b3f327e104
Depends-On: I731cc8a0f5bfeda8f17a78c33b9f44062323a361

elements/pip-and-virtualenv/README.rst

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+==================
+pip-and-virtualenv
+==================
+
+This element installs pip and virtualenv in the image. If the package
+installtype is used then these programs are installed from distribution
+packages. If the source installtype is used these programs are installed
+from get-pip.py and pip (respectively).
+
+To install pip and virtualenv from package:
+
+  export DIB_INSTALLTYPE_pip_and_virtualenv=package

elements/pip-and-virtualenv/element-deps

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+package-installs
+source-repositories

elements/pip-and-virtualenv/install.d/get-pip-py-source-install/01-install-pip

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+#!/bin/bash
+
+if [ ${DIB_DEBUG_TRACE:-0} -gt 0 ]; then
+    set -x
+fi
+set -eu
+set -o pipefail
+
+python /tmp/get-pip.py
+pip install virtualenv

elements/pip-and-virtualenv/package-installs.yaml

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+python-pip:
+  installtype: package
+python-virtualenv:
+  installtype: package

elements/pip-and-virtualenv/source-repository-pip-and-virtualenv

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+get-pip-py file /tmp/get-pip.py https://bootstrap.pypa.io/get-pip.py