Enable custom structure for group claim with default name group (#839)

* Allow complex structure for groups in group claim.

* Remove unused constant

* Update variable name

* Fix linting

* Use helper method

* Log error if not possible to append group value

* Add missing import

* Use own logger

* Fix imports

* Remove Dockerfile for testing

* Add Changelog entry

* Use formatGroup helper method and update tests

* Return string instead of string array

* Remove groups variable

* Return error in format method.

* Reorder imports

Co-authored-by: Nick Meves <nick.meves@greenhouse.io>