Writes and posts easy-to-understand Gerrit comments from the Deployment
Pipeline.

This is the alternative to the current circuitous path of
links users of the pipeline are meant to follow to find basic
information about build status, Docker images, and Docker tags.

Adds two new classes that are meant to be invoked from inside a Jenkins
job defined as a Jenkins Pipeline script. By providing access to a few
build parameters from within a Jenkins job, this patch will post a
comment to Gerrit using an output format that will be styled by Gerrit
commentstyles implemented in I5b04aa10d54b6f2587da196c02ebff9bfe5ba166.

Example posted message:

    pipeline-dashboard: service-pipeline
    pipeline-build-result: SUCCESS (job: service-pipeline, build: 27)
    IMAGE:
     docker-registry.wikimedia.org/wikimedia/mediawiki-services-citoid

    TAGS:
     test, latest

Change-Id: I2fc0924996eb1a969fcbf41bac333d3c35cd34ea