Make maven repository configurable via an env var

[msinto93]

Make Maven repository configurable via an env var, default to public Maven repo

Issue number: #209

Description of changes: Currently python setup.py download_jars is hardcoded to download the KCL jars from the public Maven repository. This change makes the Maven repository configurable via an env var (defaulting to public Maven if not set). This will allow this package to be installed directly via a pip install for anyone using their own Maven repository.

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[msinto93]

@shanmsac Would you mind reviewing this minor change please?

[msinto93]

@akidambisrinivasan @brendan-p-lynch @furq-aws Tagging you as recent reviewers on this repo, would one of you mind taking a look at this change please?