raft: replication test: support config change
raft: replication test: support config change

Add support for configuration change on leader.

Keep track of servers in config in test.

Add a dummy entry to confirm configuration changed. If the add fails,
because the old leader was not in the new config and stepped down, the
config is considered changed, too.

Add a test with some configuration changes.
Add a test cycling every scenario for 1 of 4 nodes removed.

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

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