refact: drs volume decoupling

[v-rocheleau]

Remove the DRS bridge volume from Gohan, as required for object storage.

Note that this still works with DRS/Drop-Box in local or S3 mode.

Rather than exchanging files from shared persistent volumes, gohan now downloads and uploads the files it needs over the network only.

Gohan's /ingest endpoint now accepts Drop-Box object paths:

1. Downloads the VCF to a temp location: /app/vcfs/<name>.vcf.gz
2. Generates a tabix file to a temp location: /app/vcfs/<name>.vcf.gz.tbi
3. Upload files to DRS (bridge-less)
4. Process VCFs
5. Remove temp files

Other changes

1. fix: vcf ingestion WDL workflow returns non zero code if an ingestion status has an error, fixes silent workflow errors
2. tests: drop-box is now part of the integration tests

[davidlougheed]

lgtm!