in-jvm dtest support for subnets doesn't change seed provider subnet

When using the `withSubnet` function on AbstractCluster.Builder, the newly-selected subnet is never used when setting up the SeedProvider in the constructor of InstanceConfig, which is hard-coded to 127.0.0.1. Because of this, clusters with any subnet other than 0, and gossip enabled, cannot start up as they have no seed provider in their subnet and what should be the seed (instance 1) doesn't think it is the seed.