Extend "sticky build" support to hubs without DIND

[betatim]

Currently the "sticky builds" feature only works when you use a docker-in-docker setup for your BinderHub. This issue is about removing this restriction. See #949 (comment) for some thoughts.

Currently we use the list of DIND pods to find out possible nodes that we can schedule builds on.

In order to remove the restriction that sticky builds only work when DIND is enabled we'd need a new way to find all eligible nodes. Ideally without needing cluster wide privileges.