[BB-4388] Adds k8s cluster in gitlab via terraform

Description

This MR integrates k8s cluster with Gitlab via terraform.

Supporting information

https://tasks.opencraft.com/browse/BB-4388

Testing instructions

1. Pull this MR
2. Deploy a Digital Ocean Kubernetes using Tutorraform as documented here. Make sure you are using README from the shimulch/integrate-gitlab-k8s-cluster branch.
3. You only need to complete the infrastructure provisioning (Part 3), till here. No need to provision Open edX instances.
4. On your private repo, go to the Infrastructure -> Kubernetes clusters.
5. Verify that the new cluster has connected with GitLab.

Deadline

None

Other information

Note Tested only in DigitalOcean provider at the moment.