Permit injecting entries into /etc/hosts.

We need this for the test-environment machines.

Change-Id: I499607a26e6282a9f37b91989fe6c1821d0cca9d

elements/hosts/README.md

@@ -0,0 +1,5 @@
+Permit injecting content into /etc/hosts from heat
+
+# Configuration
+
+    hosts: '12.3.4 my-host.com'

elements/hosts/element-deps

@@ -0,0 +1,2 @@
+os-apply-config
+os-refresh-config

elements/hosts/os-apply-config/var/run/hosts.d/tail

@@ -0,0 +1 @@
+{{hosts}}

elements/hosts/os-refresh-config/configure.d/51-hosts

@@ -0,0 +1,19 @@
+#!/bin/bash
+
+set -eu
+set -o pipefail
+
+# 51 - right after oac as hosts is needed for $justabouteverything.
+
+if [ ! -e /etc/hosts -o ! -e /var/run/hosts.d/tail ]; then
+    exit 0
+fi
+
+if fgrep -f /var/run/hosts.d/tail /etc/hosts; then
+    exit 0
+fi
+cp /etc/hosts /var/run/hosts.d/new-hosts
+echo >> /var/run/hosts.d/new-hosts
+cat /var/run/hosts.d/tail >> /var/run/hosts.d/new-hosts
+echo >> /var/run/hosts.d/new-hosts
+mv /var/run/hosts.d/new-hosts /etc/hosts