Shave some time off bootstrap

[codefromthecrypt]

According to @dsyer, listing config locations can speed up things that otherwise require scanning.

I tried this and it took off a couple hundred ms from the latest image:

$ docker run -d -p 9411:9411 openzipkin/zipkin java -cp . org.springframework.boot.loader.PropertiesLauncher --spring.config.location=file://./BOOT-INF/classes/zipkin-server.yml,file://./BOOT-INF/classes/zipkin-server-shared.yml

before:
Started ZipkinServer in 5.258 seconds (JVM running for 6.475)

after:
Started ZipkinServer in 5.028 seconds (JVM running for 6.217)

Trick is that you have to list out the exact paths. this would require us finding them for our "modules" such as scribe, kafka08 and also layered images like zipkin-aws and zipkin-gcp. However, they are in static locations so mostly this is composing env variables I suppose

cc @openzipkin/devops-tooling