raft: replication test: use disconnect() helper
raft: replication test: use disconnect() helper

For rpc tests, use raft_cluster::disconnect() instead of the local
connected reference.

This removes connected object use outside raft_cluster.

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

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