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.

First phase reviews: @tymofij  @olmar 
If the tests need that flag on to run, then we need a better way to set this up. If they're unit tests, you could change the value of the flag in `test.py`. Or, you could decorate the tests to override this flag and set it to `True` for the duration of the tests.