raft: group 0: use cfg.contains() for config check
raft: group 0: use cfg.contains() for config check

There will be nodes in non-voting state in configuration, so can_vote()
is not a good check. Use newer cfg.contains().

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

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