Add build-time labels to generated docker image and allow to add custom ones

[mguillem]

Currently my image built with the fabric8 maven plugin have labels like these ones even if the image has been build a few hours ago:

    build-date=20170605 
    io.fabric8.s2i.version.jolokia=1.3.6 
    io.fabric8.s2i.version.maven=3.3.3 
    io.k8s.description=Platform for building and running plain Java applications (fat-jar and flat classpath) 
    io.k8s.display-name=Java Applications 
    io.openshift.build.image=fabric8/s2i-java:2.0 
    io.openshift.build.name=svc-survey-s2i-1 
    io.openshift.build.namespace=it 
    io.openshift.build.source-location=/tmp/s2i-build511047928/upload/src 
    io.openshift.s2i.destination=/tmp 
    io.openshift.s2i.scripts-url=image:///usr/local/s2i 
    io.openshift.tags=builder,java 
    license=GPLv2 
    org.jboss.deployments-dir=/deployments 
    vendor=CentOS 

All these labels are not so interesting.

It would be great if the s2i-java builder could add labels to the generated docker image like those added to a generated deployment config (for instance fabric8.io/git-commit ) as well as custom ones.

According to this doc, it seems that this can be done by generating a /tmp/.s2i/image_metadata.json file.