Implement `HasVariable` concern for sharing `Ci::Variable` core implementation with Group/Project/PipelineSchedule Variables
Description
Implement HasVariable
concern for sharing Ci::Variable
core implementation with Group/Project/PipelineSchedule Variables
Proposal
This concern has already been implemented at https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12341. So We'll decouple the functionality and create a minimal MR.
https://gitlab.slack.com/archives/C0SFP840G/p1498475280740411