Update SystemService.diagnostics() method
Update SystemService.diagnostics() method

The `SystemService.diagnostics()` now returns a `(io.BytesIO,
'filename')` tuple allowing the caller to save the `cdrouter-diag`
contents returned by the `GET /api/v1/system/diag/` API endpoint to
the place of their choosing.  Previously, this method returned a
string, however starting in CDRouter 13.3 `cdrouter-diag` produces a
`.tar.gz` tarball rather than a `.txt` plain-text file.

qacafe/cdrouter.py,qacafe/cdrouter.py