API support for group-level clusters

Problem to solve

Building on https://gitlab.com/gitlab-org/gitlab-ce/issues/40473, provide the same level of API functionality for group-level clusters.

Target audience

developers, operators.

Further details

Proposal

Provide API endpoints where users can leverage the kubernetes integration.

Considerable API Endpoints

• Get a list of clusters; get ':group_id/clusters'

• Create a cluster; post ':group_id/clusters'

• Edit a cluster; put ':group_id/clusters/:cluster_id'

• Destory a cluster; delete ':group_id/clusters/:cluster_id'

• Install an app into a cluster; post ':group_id/clusters/:cluster_id/#{app_name}'

• Uninstall an app into a cluster; delete ':group_id/clusters/:cluster_id/#{app_name}'

What does success look like, and how can we measure that?

(If no way to measure success, link to an issue that will implement a way to measure this)

Links / references