Add contrail-repo element

This commit adds the contrail-repo element, allowing one to install
contrail packages. Contrail does not provide open repositories one needs
to get them from their Juniper Software Download page and then host them
in a reachable place for diskimage-builder to go pick it up from there.

Only works with CentOS/RHEL at the moment.

Change-Id: I7455c71ca8747ee05b5a87362e2bd6a5527f86c8

elements/contrail-repo/README.md

@@ -0,0 +1,6 @@
+Enables the Contrail repository.
+
+This element requires the $CONTRAIL_URL variable set to specify a path to a
+contrail-install-packages package to be installed.
+
+This elements currently only works on a RHEL based system.

elements/contrail-repo/element-deps

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+pip-manifest
+rdo-release

elements/contrail-repo/post-install.d/10-contrail-setup

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+#!/bin/bash
+
+set -eux
+
+/opt/contrail/contrail_packages/setup.sh

elements/contrail-repo/pre-install.d/10-contrail-repo

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+#!/bin/bash
+
+set -eux
+
+export CONTRAIL_URL=${CONTRAIL_URL:-""}
+
+if [ -z "$CONTRAIL_URL" ]; then
+  echo "One should specify an HTTP URL to the contrail-install-packages for this element to work"
+  exit 1
+fi
+
+yum -y install $CONTRAIL_URL