search: Default to k8s service discovery for indexed-search
Created by: keegancsmith
This means that admins do not need to configure INDEXED_SEARCH_SERVERS if using k8s.
Test plan: I was a chaos monkey in production. Tested how Sourcegraph.com behaved when I killed some or all of the indexed-search replicas. for pod in indexed-search-{0,1,2}; do kubectl exec -it $pod -- pkill zoekt-webserver; done
Part of https://github.com/sourcegraph/sourcegraph/issues/5725