Show correct Operating System for Red Hat Enterprise Linux 7 in motd.

The file /etc/issue looks on Red Hat Enterprise Linux 7 like this:

~~~
\S
Kernel \r on an \m
~~~

The \S gets expanded to the current release. For this reason the
existing functionality doesn't work as expected.

The File /etc/redhat-release contains the correct version string:

Red Hat Enterprise Linux Server release 7.1 (Maipo)

opsworks_stack_state_sync/recipes/motd.rb

@@ -1,9 +1,17 @@
+os_release =
+  if node[:platform] == "redhat" && Chef::VersionConstraint.new("~> 7.0").include?(node["platform_version"])
+    os_release = File.read("/etc/redhat-release").chomp
+  else
+    `head -1 /etc/issue | sed -e 's/ \\\\.*//'`.chomp
+  end
+
 template "/etc/motd.opsworks-static" do
   source "motd.erb"
   mode "0644"
   variables({
     :stack => node[:opsworks][:stack],
     :layers => node[:opsworks][:layers],
     :instance => node[:opsworks][:instance],
+    :os_release => os_release
   })
 end

opsworks_stack_state_sync/templates/default/motd.erb

@@ -1,7 +1,7 @@
  This instance is managed with AWS OpsWorks.
 
    ######  OpsWorks Summary  ######
-   Operating System: <%= `head -1 /etc/issue | sed -e 's/ \\\\.*//'`.chomp %>
+   Operating System: <%= @os_release %>
    OpsWorks Instance: <%= @instance[:hostname] %>
    OpsWorks Instance ID: <%= @instance[:id] %>
    OpsWorks Layers: <%= @instance[:layers].map{|id| @layers[id][:name] }.join(', ') %>