Allow distributionBaseUrl to be set using a system property

[mmorrisontx]

In large build systems, it would be useful for distributionBaseUrl to be overridable using a java system property or environment variable, so that the binary download can be redirected to a caching mirror without needing to touch the source of every involved project.

[michaelmosmann]

@mmorrisontx so just change the base url to an mirror, right?

[mmorrisontx]

Yep! Basically just a way to do this from the environment without needing a code change:

@Override
public Transition<DistributionBaseUrl> distributionBaseUrl() {
    return Start.to(DistributionBaseUrl.class)
    .initializedWith(DistributionBaseUrl.of("http://my.custom.download.domain"));
}

[michaelmosmann]

@mmorrisontx will do :)