Remove custom role in favour of Kubernetes Engine Cluster Viewer role

[ViliusS]

Built-in Kubernetes Engine Cluster Viewer role is very minimal and contains all permissions needed to run this plugin. This PR removes custom role from documentation and Terraform files, and replaces it with built-in role.

Previous suggested permissions:

• container.apiServices.get
• container.apiServices.list
• container.clusters.get
• container.clusters.getCredentials
• container.clusters.list
• resourcemanager.projects.get

New permissions in built-in role:

• container.clusters.connect
• container.clusters.get
• container.clusters.list
• resourcemanager.projects.get
• resourcemanager.projects.list

I think container.apiServices.* where included by mistake previously. These permissions grant access to Kubernetes level APIServices objects and should be included in RBAC if one really needs to deploy them through plugin.

container.clusters.getCredentials was also not really correct as credentials are generated dynamically, using service account API key. Kubeconfig file is never retrieved or created AFAIK.

Testing done

Configured my Service Account to only have new built-in role. Then tested both, Jenkins Declarative Pipeline and Jenkins UI. Plugin works as before.

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