unifi: Deploy unifi_exporter

`unifi_exporter` provides Prometheus metrics for UniFi controller.

roles/unifi/templates/unifi_exporter.container.j2

@@ -0,0 +1,31 @@
+[Unit]
+Description=Prometheus metrics exporter for Unifi Controller
+Wants=unifi.service
+After=unifi.service
+
+[Container]
+Image={{ unifi_exporter_container_image }}:{{ unifi_exporter_version }}
+Volume=/etc/unifi_exporter.yml:/etc/unifi_exporter.yml:ro
+Network=host
+NoNewPrivileges=yes
+Exec=-config.file /etc/unifi_exporter.yml
+User=200
+Group=200
+VolatileTmp=yes
+
+[Service]
+Restart=always
+RestartSec=2
+PrivateTmp=yes
+ProtectClock=yes
+ProtectHome=yes
+ProtectKernelModules=yes
+ProtectProc=invisible
+ProtectSystem=strict
+ReadWritePaths=/run
+ReadWritePaths=/var/lib/containers/storage
+RestrictRealtime=yes
+UMask=0077
+
+[Install]
+WantedBy=multi-user.target