Anton/sync groups and verticals

This fixes https://openedx.atlassian.net/browse/BLD-1095

NB: before merge `ENABLE_GROUP_CONFIGURATIONS` should be set to default value.

Please review: @tymofij  @cahrens 
I know what are you talking about: http://stevenloria.com/python-best-practice-patterns-by-vladimir-keleshev-notes/
It is about ease of code review. I do not like this practice. 

Fixed: return None explicitly