Add e2e test cases of managing etcd cluster

[ahrtr]

We need to add e2e test cases to verify the etcd-operator's functionalities of managing etcd clusters:

• create & delete 1 member etcd cluster
• create & delete 3 member etcd cluster
• scale in & out etcd cluster
• sanity test against the etcd cluster
  • read & write k/v
  • member list
  • health check

Kubebuilder automatically generates e2e test using ginkgo, but personally I am not a big fan of ginkgo. It creates some syntax sugars on top of golang standard test, and organize the test in a hierarchical structure. When the test file gets bigger and bigger, it will become more and more unreadable.

So it would be great if we can add the e2e test case just using the golang standard test utilities.

[abdurrehman107]

I can take this up guys.