Expose container ip address as maven properties

[changgengli]

First, this is very nice work and I'm trying to use it to set up the integration test.

The problem I have is I have to bind the port number of a service in the container to a port of the host. This would make running several tests in parallel very diffcult and the port may have conflicts with the using port number of the host.

The solution I'm thinking is to expose the container's ip address as maven properties.
I just forked this repository and the orchestration-core, and add the support of this.

My changes and idea can be seen here:
http://git.io/vi5aeg
http://git.io/zDsWOA

This doesn't provide a generic way to expose any properties of the container, just the ip address.
If you think this is reasonable I can create a PR for this.

[alexec]

Looks excellent! I'd like to suggest a couple of small changes:

• Convention over configuration: can we always expose them?
• Command query separation: rather than return the map from the start method, provide a new methods called something like 'getIps'?
• Maybe add a prefix to the property name to avoid collision? E.g. 'Docker.example-project_app.ip'