Add tests for operator

We recently found a nasty bug where IPv6 records would not get generated when there were multiple router nodes in a cluster. This failure case was not caught by our "small" clusters we use for integration tests. !7 (merged)

Thus, we should set up some tests in a "proper" environment:

• build the image with branch tag
• deploy this operator (container) to paas-stg
• run the operator in dry-run mode (do not update any records)
• check that the generated nsupdate file is valid