infra/gcp/prow: Add e2e project for sig-scalability
infra/gcp/prow: Add e2e project for sig-scalability

Add e2e project for sig-scalability prowjobs that need 5K nodes.
Initial suggestion : https://github.com/kubernetes/test-infra/pull/22430#pullrequestreview-677953930

Signed-off-by: Arnaud Meukam <96190fcd0798f0afa72279990e31ddc33e4f2ea2@gmail.com>

kubernetes/k8s.io,kubernetes/k8s.io,kubernetes/k8s.io,kubernetes/k8s.io