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 
It concerns me that we have tests that only run if you know to temporarily override this variable. I worry that other developers won't know to run the tests these way, and thus will inadvertently break the group configuration functionality.