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 
Best practice is to be explicit when returning `None`, rather than just relying on falling out of the end of the method to do it, if other places in your method also return a value.