Add disk usage to release.py script

[pwalczysko]

See suggestion on https://github.com/IDR/SubmissionWorkflow/pull/25#discussion_r485778995

Explore the feasibility of adding the lines

    export PGPASSWORD=$(/opt/omero/server/OMERO.server/bin/omero config get omero.db.pass)
    export PGHOST=$(/opt/omero/server/OMERO.server/bin/omero config get omero.db.host)
    psql -U omero idr -c "select pg_database_size('idr')"

into https://github.com/IDR/idr-utils/blob/master/scripts/releases.py

cc @manics

[sbesson]

Generally makes sense to compute as much as possible. Main caveat is that the script would need to be run from the deployment while it can be executed locally currently but we can live with this trade-off