raft: replication test: add etcd test for cycling leaders
raft: replication test: add etcd test for cycling leaders

This test cycles 3 nodes as leaders without adding entries.

Signed-off-by: Alejo Sanchez <004be06a28e38b437b038f9b5c0b9f5758b792e9@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla