raft: replication test: raft_cluster add_entries
raft: replication test: raft_cluster add_entries

Move add_entries() to raft_cluster and provide a helper to add remaining
entries.

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

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