Drop obsolete synccloud management command

NC-202
Drop obsolete Cloud sync method and REST action

NC-202
Rename Flavor's flavor_id to backend_id

Done to keep consistency  with the rest of models.
Merge branch 'feature/nc-202' into 'develop'

Cloud sync remnants cleanup, small flavor db change NC-202

Dropped obsolete ways to synchronize a cloud: `synccloud` management command and `/api/clouds/ID/sync/` endpoint.
Renamed `Flavor`'s `flavor_id` to `backend_id` to match with the rest of the code.