status: use env as parameters to service

Instead of having to rewrite the service file with parameters,
leverage Env file to pass default parameters values.

systemd/obs-status-service.service

@@ -0,0 +1,16 @@
+[Unit]
+Description=OBS build status as SVG service
+After=network-online.target
+
+[Service]
+Type=exec
+Restart=on-failure
+ExecStart=/usr/bin/obs-status-service
+EnvironmentFile=-/etc/default/obs-status-service.env
+DynamicUser=yes
+NoNewPrivileges=yes
+ProtectSystem=strict
+
+[Install]
+WantedBy=multi-user.target
+