Add credential storage (#6938)

* add credential store interface

* load credential.toml

* lint

* create credentialManager with explicit store type

* add type name

* InitializeCredentialManager

* remove unused functions

* fix missing import

* fix import

* fix nil configuration

weed/command/s3.go

@@ -5,7 +5,6 @@ import (
 	"crypto/tls"
 	"crypto/x509"
 	"fmt"
-	"github.com/seaweedfs/seaweedfs/weed/util/version"
 	"io/ioutil"
 	"net"
 	"net/http"
@@ -14,6 +13,8 @@ import (
 	"strings"
 	"time"
 
+	"github.com/seaweedfs/seaweedfs/weed/util/version"
+
 	"github.com/seaweedfs/seaweedfs/weed/s3api/s3err"
 	"google.golang.org/grpc/credentials/tls/certprovider"
 	"google.golang.org/grpc/credentials/tls/certprovider/pemfile"