roles/haproxy: Basic setup for HAproxy

The *haproxy* installs HAproxy and sets up basic configuration for it.
It configures the systemd unit to launch the service with the `-f
/etc/haproxy` arguments, which will cause it to load all files from the
`/etc/haproxy` directory, instead of just `/etc/haproxy/haproxy.cfg`.
This will allow other roles to add frontend and backend configuration by
adding additional files to this directory.

roles/haproxy/files/haproxy-config.conf

@@ -0,0 +1,2 @@
+[Service]
+Environment=CONFIG=/etc/haproxy