Add ability to configure working directory of ETL

[rbreslow]

As it's currently designed, the ETL will write files to the current working directory (which is /usr/local/src). To take advantage of ephemeral storage on AWS Batch, we need to add a mechanism that tells the ETL to perform operations in an alternative directory (e.g., /media/ephemeral0).

Related discussion: https://github.com/d3b-center/image-deid-etl/pull/34/files#r872752946