Check for Environment Scope in ReadVariable

Created by: oesah

I am coming here from https://github.com/terraform-providers/terraform-provider-gitlab.

Problem We are using go-gitlab to create resources using terraform. We face an issue, however, where project or group variables do not consider the scope. So if you have 2 variables with the same name but different scopes, it will cause terraform to always try to update them. See discussion here: https://github.com/terraform-providers/terraform-provider-gitlab/issues/213

I think the issue lie here: https://github.com/xanzy/go-gitlab/blob/master/group_variables.go#L84 https://github.com/xanzy/go-gitlab/blob/master/project_variables.go#L86

Possible Solutions Currently, the Gitlab API does not allow to filter them. You can follow the issue here: gitlab-org/gitlab#20661 (closed)

Once that bug is fixed, we can enhance the above mentioned functions to incorporate the filtering solution.