storage_service: allow a node to join without bootstrapping
storage_service: allow a node to join without bootstrapping

... even if it couldn't contact other nodes.

This reverts 7cb6ac33f5f54cae665cecd3a90428eb52d424e3.

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