Add `release package` subcommand.

Summary:
* Generates a metadata json file for each build, as `$pkg.metadata.json`
* Adds `release` namespace to CLI tool, and `release package` command which uploads package and metadata files to S3
* Upgrades fpm to 0.4.33; Use fpm internal API to determine name for fpm packages. This is needed for metadata generation, has been tested with a chef-client omnibus build in Ci.
* Omnibus CLI will now always attempt to load a config file when there's a `:config` option present. This is done because the release command doesn't need or care about a project root, but it does use the config file to get S3 configuration.