EKS Clusters: Select cloud provider buttons

What does this MR do?

This MR is part of delivering #22392 (closed). It does not deliver the whole feature. It only delivers the functionality described below:

Displays Google GKE and Amazon EKS logos in the buttons to select a cloud provider to create a K8 cluster

When the user clicks Amazon EKS button, it displays Create EKS Cluster form. It changes the tab title to "Create new Cluster on EKS"

When the user clicks "Google GKE", it should be redirected to the Create GKE Cluster form.

No user-facing changes

• Splits and extracts GKE Cluster template into individual HAML partials for easier reuse.
• Implements E2E test that ensures a user can open the "Create EKS cluster" form.

Testing

Before testing

• Enable create_eks_clusters feature flag
• Set up Google OAuth https://gitlab.com/gitlab-org/gitlab-development-kit/blob/master/doc/howto/google-oauth2.md

How to test

• In the sidebar menu, select Operations -> Kubernetes
• Add Kubernetes cluster button.