Revert "Revert Xenial to Python 2"

We landed the fix for this in
Icdb769541eee9793f261b4b8ec563be76ee13fe2.

This reverts commit 2978ff885b.

Change-Id: Iecfc41ab2aad57bc4f6f86a13810b534d19a8fd5

elements/dib-python/environment.d/50-dib-python-version

@@ -5,8 +5,6 @@ if [ -z "${DIB_PYTHON_VERSION:-}" ]; then
             DIB_PYTHON_VERSION=2
         elif [ "$DIB_RELEASE" == "trusty" ]; then
             DIB_PYTHON_VERSION=2
-        elif [ "$DIB_RELEASE" == "xenial" ]; then
-	    DIB_PYTHON_VERSION=2
         fi
     elif [ "$DISTRO_NAME" == "debian" ]; then
         DIB_PYTHON_VERSION=2

elements/ubuntu/pre-install.d/00-install-python

@@ -1,10 +0,0 @@
-#!/bin/bash
-if [ ${DIB_DEBUG_TRACE:-0} -gt 0 ]; then
-    set -x
-fi
-set -eu
-set -o pipefail
-
-# hack to get python into Xenial python3 only images, while we
-# work-around issues.
-apt-get -y install python || true