[Review] Request from 'b4mboo' @ 'SUSE/connect/review_140905_write_config'

Based this [Trello card](https://trello.com/c/ophfaMfz/2182-p1-bnc-894853-invalid-system-credentials-suseconnect-does-not-write-config), please review the following changes:
  * 56f3efc introduce --write-config
  * 0093a8b adjust feature syntax