Adds .bosh_config option for max_parallel_downloads

Defaults to 1 (current behavior), will read the file and downloads blobs
in parallel if configuration is set.
Add --max-parallel-downloads (-M) option to CLI

This overrides the max_parallel_downloads config and takes precedence.

You can now:

    bosh -M 5 sync blobs
    OR
    bosh -M 5 create release