Ensure haproxy.cfg ends with newline

Versions of Haproxy >=2.3 require the config file to end in a newline
or they'll exit with an error.

Change-Id: I9301ea679536b10ee5ad0d87d42c1655e5852616

promenade/templates/roles/common/etc/promenade/haproxy/haproxy.cfg

@@ -42,3 +42,5 @@ backend kube-system-kubernetes-etcd-be
   {%- for ip in config.join_ips %}
   server s{{ ip }} {{ ip }}:{{ port }} check port {{ port }}
   {%- endfor %}
+
+# Haproxy requires config files end with a newline.