promote bucket to a const
if AUTOPARTS_HOST exists, use it as BINARY_HOST

when the environment specifies AUTOPARTS_HOST env, we use it as the package
host. This is useful to test changes for packages that we dont want to promote
to production rightaway.
BINARY_BUCKET -> AUTOPARTS_BUCKET if exists

AUTOPARTS_BUCKET should be the s3 staging bucket
Merge pull request #71 from nitrous-io/staging-bucket

Replace s3 buckets and binary host when we want to get packages from/to staging buckets