Support meaningful discovery instance id in containerized envrionment

[stgrosshh]

In containerized environment we cannot use server.port, when having
more than one service instances running on one host. Introduce first
container related property to build a instance id with mapped host port
to be registered in discovery

[spencergibb]

Help me understand your use case. What discovery server are you using (eureka, consul, zookeeper)? You can already customize the instance id's in each technology.

For example in a eureka app in application.properties (or bootstrap.properties)

eureka.instance.instanceId=${CONTAINER_HOST_PORT:${spring.application.name}:${server.port}}

Would set the instanceId to CONTAINER_HOST_PORT if set, otherwise spring.application.name:server.port for local dev.

[stgrosshh]

Yeah, that's an alternative approach, of course.
Thought it would be a possibly more convenient default though.
I was a little bit puzzled in the first place, when setting the eureka.instance.nonSecurePort, that it was
not used in the instance id. So I dogged a little bit in the code and came to the suggested approach.

At least some clarification regarding this would be great in the documentation.

[spencergibb]

IdUtils was created to create a default that might be useful on a laptop. It is documented how to change the instance id http://cloud.spring.io/spring-cloud-static/Brixton.SR5/#_changing_the_eureka_instance_id.

The simplest way to manually make instance id unique would be to set spring.application.instance_id.

I feel like this is more complex than the already allowed configuration.