Appsettings not mapping to Options class

Program

` is defined as such:
```cs
        public static IWebHost InitializeIdentityDb<TUser, TRole>(this IWebHost host)
        {
            using (var scope = host.Services.CreateScope())
            {
                var services = scope.ServiceProvider;
                var options = services.GetRequiredService<IOptions<CassandraOptions>>();
...
                var initializer = new DbInitializer(session);
                initializer.Initialize<TUser, TRole>(options.Value);

                return host;
            }
        }
```
Then 

 is declared as:
```cs
    public class CassandraOptions
    {
        public List<string> ContactPoints { get; set; }
        public CassandraCredentials Credentials { get; set; }
        public string KeyspaceName { get; set; }
        public Dictionary<string, string> Replication { get; set; } = null;
        public bool DurableWrites { get; set; } = true;
        public CassandraQueryOptions Query { get; set; }
    }
```
Now at my breakpoint on 

, I see that 

 is not populated. Colleagues reckon it's supposed to be populated from command line arguments, but how do I define nested values like