Part of ENG-4573. Added a new --s3-bucket-name CLI option and BLOB_S3_BUCKET_NAME environment variable that allows to override the default commapp-blob S3 bucket. We need this to be able to use a separate bucket in staging environment.

Ran integration tests locally - confirmed that default S3 bucket is used. Ran integration tests with --s3-bucket-name set to non-existent bucket - they fail with NoSuchBucketError as expected. Repeated with the environment variable set instead.