ENYO-2949: better specify `overwrite: undefined/true/false`

- `overwrite != false` means `overwrite = true`
- add file-system test cases
- set file-manager operations create/copy/move to `overwrite=false`

Enyo-DCO-1.1-Signed-off-by: Francois-Xavier KOWALSKI <francois-xavier.kowalski@hp.com>