test_cases.md

* ci: redefine multinode to node-etcd-client

This should allow to catch several class of problem rather than just
one -> from network plugin such as calico or cilium talking directly to
the etcd.

* Dynamically define etcd host range

This has two benefits:
- We don't play the etcd role twice for no reason
- We have access to the whole cluster (if needed) to use things like
  group_by.