test: raft: randomized_nemesis_test: send `modify_config` requests in reconfiguration nemsesis
test: raft: randomized_nemesis_test: send `modify_config` requests in reconfiguration nemsesis

Extend the reconfiguration nemesis to send `modify_config` requests as
well as `reconfigure` requests. It chooses one or the other with
probability 1/2.

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