Add integration test, for real

[marcosvm]

👻

@bejelith

This PR adds other service on the composer docker_entrypoint_integration that will invoke the created executable with the following command line:

/docker_entrypoint -template /etc/server/server.properties.template /bin/cat /etc/server/server.properties

Thus the integration test will fail if the /etc/server/server.properties is not generated. The result of /bin/cat /etc/server/server.properties can be used for visual inspection of the generated file.

The environment variables for substitution in the template are defined in the Dockerfile for the test image test/Dockerfile.test.

vai.sh now run two docker-compose up commands as I could not find a way to fail the test if one of the services failed, docker-compose as returning exit code 0 if so. The drawback is that the test image is built twice but that can be a good thing as isolated both unit and integration tests.

Travis is passing.