Use memory storage for etcd in e2d2 test clusters

[pablochacin]

etcd is a well know source of overhead when running test clusters with kind.

The most common approach is to use tmpfs for the etcd database:

kind: Cluster
apiVersion: kind.x-k8s.io/v1alpha4
kubeadmConfigPatches:
- |
  kind: ClusterConfiguration
  etcd:
    local:
      dataDir: /tmp/etcd     # /tmp is mapped to tmpfs in kind's nodes

This configuration option could be added to the e2ecluster to enable faster cluster operation.