Change how gitlab-group is parsed on options (#639)

* Changed how gitlab-group is parsed, from string to []string

See #637

* Point out that gitlab-group can be a list

See #637

* Reflect to the []string change on pkg/apis/options/options.go

See #637

* Move cfg option gitlab_group to gitlab_groups

See #637

* Renamed Group to Groups

See #637

* Reflect the change on gitlab.go as well

See #637

* Added #639

* Added the author of #639 to the CHANGELOG

* Add the gitlab_groups env change to CHANGELOG.md

See #639

Co-authored-by: Joel Speed <Joel.speed@hotmail.co.uk>

Co-authored-by: Joel Speed <Joel.speed@hotmail.co.uk>