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/bucket-handlers.go

@@ -49,6 +49,10 @@ func enforceBucketPolicy(bucket, action, resource, referer string, queryParams u
 		return ErrInternalError
 	}
 
+	if globalBucketPolicies == nil {
+		return ErrAccessDenied
+	}
+
 	// Fetch bucket policy, if policy is not set return access denied.
 	policy := globalBucketPolicies.GetBucketPolicy(bucket)
 	if policy == nil {