Add basic integration testing

[reneradoi]

This PR adds a basic integration test to the etcd operator which:

• deploys a cluster of three etcd nodes
• checks the cluster's availability
• checks if all deployed nodes where added as cluster members
• writes test data to etcd
• reads the test data

[skourta]

Thank you, René. This should get us started with the integration tests. I left enhancement comments but it already LGTM.

[reneradoi]

Thank you for your feedback. We can address the comments when we further enhance the tests. Of course we need to add more tests and more functionality to the testing suite. This PR here is just considered a basic coverage to get fast feedback when making changes to the code, and to give a starting point on how to create the tests.