storage: add etcd storage

This patch adds etcd storage implementation. This should be useful in
environments where
- we dont want to depends on a separate, hard to maintain SQL cluster
- we dont want to incur the overhead of talking to kubernetes apiservers
- kubernetes is not available yet, or if kubernetes depends on dex
to perform authentication and the operator would like to remove any
circular dependency if possible.

cmd/dex/config.go

@@ -11,6 +11,7 @@ import (
 
 	"github.com/coreos/dex/server"
 	"github.com/coreos/dex/storage"
+	"github.com/coreos/dex/storage/etcd"
 	"github.com/coreos/dex/storage/kubernetes"
 	"github.com/coreos/dex/storage/memory"
 	"github.com/coreos/dex/storage/sql"
@@ -124,6 +125,7 @@ type StorageConfig interface {
 }
 
 var storages = map[string]func() StorageConfig{
+	"etcd":       func() StorageConfig { return new(etcd.Etcd) },
 	"kubernetes": func() StorageConfig { return new(kubernetes.Config) },
 	"memory":     func() StorageConfig { return new(memory.Config) },
 	"sqlite3":    func() StorageConfig { return new(sql.SQLite3) },