raft: replication test: make initial leader just default
raft: replication test: make initial leader just default

The test suite requires an initial leader and at the moment it's always
just 0. Make it default and simplify code.

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

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