Implement S3 Gateway to third party cloud storage providers. (#3756)

Currently supported backend is Azure Blob Storage.

```
export MINIO_ACCESS_KEY=azureaccountname
export MINIO_SECRET_KEY=azureaccountkey
minio gateway azure
```

cmd/gateway-router.go

@@ -0,0 +1,122 @@
+/*
+ * Minio Cloud Storage, (C) 2017 Minio, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package cmd
+
+import (
+	"io"
+
+	router "github.com/gorilla/mux"
+)
+
+// GatewayLayer - Interface to implement gateway mode.
+type GatewayLayer interface {
+	ObjectLayer
+	AnonGetObject(bucket, object string, startOffset int64, length int64, writer io.Writer) (err error)
+	AnonGetObjectInfo(bucket, object string) (objInfo ObjectInfo, err error)
+	SetBucketPolicies(string, []BucketAccessPolicy) error
+	GetBucketPolicies(string) ([]BucketAccessPolicy, error)
+	DeleteBucketPolicies(string) error
+	AnonListObjects(bucket, prefix, marker, delimiter string, maxKeys int) (result ListObjectsInfo, err error)
+	AnonGetBucketInfo(bucket string) (bucketInfo BucketInfo, err error)
+}
+
+// Implements and provides http handlers for S3 API.
+// Overrides GetObject HeadObject and Policy related handlers.
+type gatewayAPIHandlers struct {
+	objectAPIHandlers
+	ObjectAPI func() GatewayLayer
+}
+
+// registerAPIRouter - registers S3 compatible APIs.
+func registerGatewayAPIRouter(mux *router.Router, gw GatewayLayer) {
+	// Initialize API.
+	api := gatewayAPIHandlers{
+		ObjectAPI: func() GatewayLayer { return gw },
+		objectAPIHandlers: objectAPIHandlers{
+			ObjectAPI: newObjectLayerFn,
+		},
+	}
+
+	// API Router
+	apiRouter := mux.NewRoute().PathPrefix("/").Subrouter()
+
+	// Bucket router
+	bucket := apiRouter.PathPrefix("/{bucket}").Subrouter()
+
+	/// Object operations
+
+	// HeadObject
+	bucket.Methods("HEAD").Path("/{object:.+}").HandlerFunc(api.HeadObjectHandler)
+	// CopyObjectPart
+	bucket.Methods("PUT").Path("/{object:.+}").HeadersRegexp("X-Amz-Copy-Source", ".*?(\\/|%2F).*?").HandlerFunc(api.CopyObjectPartHandler).Queries("partNumber", "{partNumber:[0-9]+}", "uploadId", "{uploadId:.*}")
+	// PutObjectPart
+	bucket.Methods("PUT").Path("/{object:.+}").HandlerFunc(api.PutObjectPartHandler).Queries("partNumber", "{partNumber:[0-9]+}", "uploadId", "{uploadId:.*}")
+	// ListObjectPxarts
+	bucket.Methods("GET").Path("/{object:.+}").HandlerFunc(api.ListObjectPartsHandler).Queries("uploadId", "{uploadId:.*}")
+	// CompleteMultipartUpload
+	bucket.Methods("POST").Path("/{object:.+}").HandlerFunc(api.CompleteMultipartUploadHandler).Queries("uploadId", "{uploadId:.*}")
+	// NewMultipartUpload
+	bucket.Methods("POST").Path("/{object:.+}").HandlerFunc(api.NewMultipartUploadHandler).Queries("uploads", "")
+	// AbortMultipartUpload
+	bucket.Methods("DELETE").Path("/{object:.+}").HandlerFunc(api.AbortMultipartUploadHandler).Queries("uploadId", "{uploadId:.*}")
+	// GetObject
+	bucket.Methods("GET").Path("/{object:.+}").HandlerFunc(api.GetObjectHandler)
+	// CopyObject
+	bucket.Methods("PUT").Path("/{object:.+}").HeadersRegexp("X-Amz-Copy-Source", ".*?(\\/|%2F).*?").HandlerFunc(api.CopyObjectHandler)
+	// PutObject
+	bucket.Methods("PUT").Path("/{object:.+}").HandlerFunc(api.PutObjectHandler)
+	// DeleteObject
+	bucket.Methods("DELETE").Path("/{object:.+}").HandlerFunc(api.DeleteObjectHandler)
+
+	/// Bucket operations
+
+	// GetBucketLocation
+	bucket.Methods("GET").HandlerFunc(api.GetBucketLocationHandler).Queries("location", "")
+	// GetBucketPolicy
+	bucket.Methods("GET").HandlerFunc(api.GetBucketPolicyHandler).Queries("policy", "")
+	// GetBucketNotification
+	bucket.Methods("GET").HandlerFunc(api.GetBucketNotificationHandler).Queries("notification", "")
+	// ListenBucketNotification
+	bucket.Methods("GET").HandlerFunc(api.ListenBucketNotificationHandler).Queries("events", "{events:.*}")
+	// ListMultipartUploads
+	bucket.Methods("GET").HandlerFunc(api.ListMultipartUploadsHandler).Queries("uploads", "")
+	// ListObjectsV2
+	bucket.Methods("GET").HandlerFunc(api.ListObjectsV2Handler).Queries("list-type", "2")
+	// ListObjectsV1 (Legacy)
+	bucket.Methods("GET").HandlerFunc(api.ListObjectsV1Handler)
+	// PutBucketPolicy
+	bucket.Methods("PUT").HandlerFunc(api.PutBucketPolicyHandler).Queries("policy", "")
+	// PutBucketNotification
+	bucket.Methods("PUT").HandlerFunc(api.PutBucketNotificationHandler).Queries("notification", "")
+	// PutBucket
+	bucket.Methods("PUT").HandlerFunc(api.PutBucketHandler)
+	// HeadBucket
+	bucket.Methods("HEAD").HandlerFunc(api.HeadBucketHandler)
+	// PostPolicy
+	bucket.Methods("POST").HeadersRegexp("Content-Type", "multipart/form-data*").HandlerFunc(api.PostPolicyBucketHandler)
+	// DeleteMultipleObjects
+	bucket.Methods("POST").HandlerFunc(api.DeleteMultipleObjectsHandler)
+	// DeleteBucketPolicy
+	bucket.Methods("DELETE").HandlerFunc(api.DeleteBucketPolicyHandler).Queries("policy", "")
+	// DeleteBucket
+	bucket.Methods("DELETE").HandlerFunc(api.DeleteBucketHandler)
+
+	/// Root operation
+
+	// ListBuckets
+	apiRouter.Methods("GET").HandlerFunc(api.ListBucketsHandler)
+}