regen-memo-files: use the --db argument when supplied

[sbesson]

The --db option was introduced to bypass the internal logic parsing the YAML configuration file and specify the PSQL connection URI directly via command-line.

As noticed during an upgrade with @erindiel, when --db is set, its value is ignored internally in the current release of the micro-service and the connection string is always set to postgresql://omero:omero@localhost:5432/omero.

Ideally testing this should happen in an environment where the connection string needs to overwritten e.g. to percent encode a character in the URI. Without this PR, ./regen-memo-files.sh --db postgresql://<db_user>:<db_pass>@<db_host>:<db_port>/<db_name> should fail at the CSV creation step. With this PR included, the connection should be successful.

Alternatively, in an environment where the database is stored using the default values (localhost, omero/omero),
./regen-memo-files.sh --db postgresql://omero:omero@localhost:5432/does_not_exist should unexpectedly complete without this PR included but fail as expected with this PR included.

[erindiel]

This is consistent with the changes we made during the upgrade, which resolved the issue.