Enable multiple GitLab configurations

Right now we can only configure one GitLab URL with its related information:
```
gitlab {
  url = ${?GITLAB_URL}
  user = ${?GITLAB_USER}
  token = ${?GITLAB_TOKEN}
  group-ids = ${?GITLAB_GROUP_IDS}
  ignored-repositories = []
}
```
We need to find a way to have multiple GitLab servers