Allow any value for gossip protocol DNS discovery domain

Currently, the `gossip_dns_host` is derived from the `FLY_APP_NAME` environment variable.

This makes it almost impossible to use the gossip protocol in an environment other than Fly, despite the implementation being pretty generic and only requiring DNS.

By allowing the `gossip_dns_host` to be specified in full and falling back to the current derivation from `FLY_APP_NAME`, it should allow other implementations of a distributed system. For instance, headless services in Kubernetes could be used to facilitate the gossip protocol.