etcd bootstrap script assume sh exists which is not the case in new etcd image distributed by EKS-D

The new etcd images appear to be built with distroless or similar base image that doesn't have `sh`, while our [etcd bootstrap script](https://github.com/awslabs/kubernetes-iteration-toolkit/blob/81e33bd66a76ef43028b72b3692f686e3c1e0288/operator/pkg/controllers/etcd/bootstrapconfig.go#L61-L94) relies on it.

We can't upgrade etcd to newer image tag until we figure out how are we going to deal with the etcd bootstrap script.

Start with operator/pkg/controllers/etcd/bootstrapconfig.go, especially lines 61-94, and inspect how the bootstrap script is executed in the etcd image. Determine an approach that works when the newer EKS-D etcd image lacks sh; done means the bootstrap succeeds with that image and the etcd upgrade can proceed.

Written by the indexing model from the issue text.