`generic-file-importer`: Server should define sourcesystem value

Issue relates to the data import module using Python scripts. Currently, both server and Python script must independently construct the `sourcesystem_cd` using the same algorithm, which can lead to inconsistencies.

Currently, `sourcesystem_cd` is built from various components:
- Script ID
- Script version
- File UUID
- etc.

The server uses this identifier to delete imported data from database. Rows are identified by combination of `encounter_id` and `sourcesystem_cd`

**Problem**:
* Both Python script and server must use identical algorithm to build `sourcesystem_cd`
* If they don't match, data deletion becomes impossible (except directly via database)

**Proposed solution**:
* Server generates `sourcesystem_cd` for each imported file
* Store in existing `properties`
* Pass `sourcesystem_cd` to script as environment variable (similar to other server information)
* Remove algorithm responsibility from Python script
* Eliminate potential discrepancies in identifier generation