New token allocator is disabled with DSE

[adejanovski]

The new token allocator uses the allocate_tokens_for_local_replication_factor setting to perform smart token allocations during bootstrap.

While this setting exists by default in Cassandra 4, it's not the case in DSE 6.8.
We need to set it explicitly to 3 (using the same default as Cassandra) unless the setting is overridden in the manifest.

Currently we end up with balance issues when using a small number of tokens.