Add haproxy confd templates

extras/README.md

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+### confd
+
+`confd` directory contains haproxy template files for the [confd](https://github.com/kelseyhightower/confd) -- lightweight configuration management tool
+You need to copy content of `confd` directory into /etcd/confd and run confd service:
+```bash
+$ confd -prefix=/service/$PATRONI_SCOPE -backend etcd -node $PATRONI_ETCD_HOST -interval=10
+```
+It will periodically update haproxy.cfg with the actual list of Patroni nodes from `etcd` and "reload" haproxy when it is necessary.
+
+
+### startup-scripts
+
+`startup-scripts` directory contains startup scripts for various OSes and management tools for Patroni.

extras/confd/conf.d/haproxy.toml

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+[template]
+#prefix = "/service/batman"
+#owner = "haproxy"
+#mode = "0644"
+src = "haproxy.tmpl"
+dest = "/etc/haproxy/haproxy.cfg"
+
+check_cmd = "/usr/sbin/haproxy -c -f {{ .src }}"
+reload_cmd = "haproxy -f /etc/haproxy/haproxy.cfg -p /var/run/haproxy.pid -D -sf $(cat /var/run/haproxy.pid)"
+
+keys = [
+    "/members/",
+]

extras/confd/templates/haproxy.tmpl

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+global
+	maxconn 100
+
+defaults
+	log	global
+	mode	tcp
+	retries 2
+	timeout client 30m
+	timeout connect 4s
+	timeout server 30m
+	timeout check 5s
+
+frontend master_postgresql
+	bind *:5000
+	default_backend backend_master
+
+frontend replicas_postgresql
+	bind *:5001
+	default_backend backend_replicas
+
+backend backend_master
+	option httpchk OPTIONS /master
+{{range gets "/members/*"}}	server {{base .Key}} {{$data := json .Value}}{{base (replace (index (split $data.conn_url "/") 2) "@" "/" -1)}} maxconn 100 check port {{index (split (index (split $data.api_url "/") 2) ":") 1}}
+{{end}}
+backend backend_replicas
+	option httpchk OPTIONS /replica
+{{range gets "/members/*"}}	server {{base .Key}} {{$data := json .Value}}{{base (replace (index (split $data.conn_url "/") 2) "@" "/" -1)}} maxconn 100 check port {{index (split (index (split $data.api_url "/") 2) ":") 1}}
+{{end}}