Extract DocSiteBuilder::Helpers#distro_map into _distro_map.yml file.

@nhr Please review.

Note that I did not add caching for the distro_map in the same way it isn't being done for the build_config.  The original code was hardcoded, so caching is probably ok, but I'm not sure I should for the same reasons outlined by the build_config note.  distro_map gets called many times, so it's hitting the yaml file on disk more than necessary, however, in reality, the timings for rake build were unaffected.