Move argocd-vault-plugin files to gitlab argocd definition

[mathis-marcotte]

follow up of #1884
The reason I went with the solution with files in the manifests repo was because I was more familiar with that repo, and I don't have rights to run the tf apply command in the gitlab repo. So it was a lot more easy for testing for me. In the future, I think we should add these components where our ArgoCD instance is initially defined in gitlab. This would keep our argocd install all in one place. This would also mirror how CNS did the install. Plus, I have seen some issues in my testing where the argo instance would get re-created, and it would then ignore the deployment override defined in the manfest repo and therefore wouldn't have the plugin installed. So having everything in one place would solve this potential issue