refactor(etcd): Init etcd client before registry.New
feat(etcd): Define EtcdServers in coreinit config
feat(etcd): Add --endpoint flag to corectl

The --endpoint flag allows a custom API endpoint to be used. The coreinit
API endpoint is just etcd.
Merge pull request #39 from bcwaldon/etcd-server

Configure etcd server coreinit and corectl should hit