Ensure Zipkin is disabled by default

Also note: We use `ZIPKIN_HOST` now, as `ZIPKIN` did not work same as in Go apps, so Java apps are gonna have to be different

src/main/resources/application.properties

@@ -1,7 +1,8 @@
 server.port=${port:8080}
 spring.rabbitmq.host=rabbitmq
 endpoints.health.enabled=false
-spring.zipkin.baseUrl=http://${zipkin:zipkin}:9411/
+spring.zipkin.baseUrl=http://${zipkin_host:zipkin}:9411/
+spring.zipkin.enabled=${zipkin_enabled:false}
 spring.sleuth.sampler.percentage=1.0
 spring.application.name=shipping
 # Disable actuator metrics endpoints