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.

storage/etcd/config.go

@@ -0,0 +1,92 @@
+package etcd
+
+import (
+	"time"
+
+	"github.com/coreos/dex/storage"
+	"github.com/coreos/etcd/clientv3"
+	"github.com/coreos/etcd/clientv3/namespace"
+	"github.com/coreos/etcd/pkg/transport"
+	"github.com/sirupsen/logrus"
+)
+
+var (
+	defaultDialTimeout = 2 * time.Second
+)
+
+// SSL represents SSL options for etcd databases.
+type SSL struct {
+	ServerName string
+	CAFile     string
+	KeyFile    string
+	CertFile   string
+}
+
+// Etcd options for connecting to etcd databases.
+// If you are using a shared etcd cluster for storage, it might be useful to
+// configure an etcd namespace either via Namespace field or using `etcd grpc-proxy
+// --namespace=<prefix>`
+type Etcd struct {
+	Endpoints []string
+	Namespace string
+	Username  string
+	Password  string
+	SSL       SSL
+}
+
+// Open creates a new storage implementation backed by Etcd
+func (p *Etcd) Open(logger logrus.FieldLogger) (storage.Storage, error) {
+	return p.open(logger)
+}
+
+func (p *Etcd) open(logger logrus.FieldLogger) (*conn, error) {
+	cfg := clientv3.Config{
+		Endpoints:   p.Endpoints,
+		DialTimeout: defaultDialTimeout * time.Second,
+		Username:    p.Username,
+		Password:    p.Password,
+	}
+
+	var cfgtls *transport.TLSInfo
+	tlsinfo := transport.TLSInfo{}
+	if p.SSL.CertFile != "" {
+		tlsinfo.CertFile = p.SSL.CertFile
+		cfgtls = &tlsinfo
+	}
+
+	if p.SSL.KeyFile != "" {
+		tlsinfo.KeyFile = p.SSL.KeyFile
+		cfgtls = &tlsinfo
+	}
+
+	if p.SSL.CAFile != "" {
+		tlsinfo.CAFile = p.SSL.CAFile
+		cfgtls = &tlsinfo
+	}
+
+	if p.SSL.ServerName != "" {
+		tlsinfo.ServerName = p.SSL.ServerName
+		cfgtls = &tlsinfo
+	}
+
+	if cfgtls != nil {
+		clientTLS, err := cfgtls.ClientConfig()
+		if err != nil {
+			return nil, err
+		}
+		cfg.TLS = clientTLS
+	}
+
+	db, err := clientv3.New(cfg)
+	if err != nil {
+		return nil, err
+	}
+	if len(p.Namespace) > 0 {
+		db.KV = namespace.NewKV(db.KV, p.Namespace)
+	}
+	c := &conn{
+		db:     db,
+		logger: logger,
+	}
+	return c, nil
+}