added checks for sn duplicates in AddPart + test
removed ending blank lines
Merge branch 'develop' of https://github.com/allegro/ralph_assets into develop
Merge branch 'develop' into sn-in-addpart
create device_info if none in _update_device_info
rm hardcoded id for device in test
Merge pull request #272 from xliiv/sn-in-addpart

Sn in addpart