Implement ServerConfig admin REST API (#3741)

Returns a valid config.json of the setup. In case of distributed
setup, it checks if quorum or more number of nodes have the same
config.json.

pkg/madmin/config-commands.go

@@ -0,0 +1,49 @@
+/*
+ * 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 madmin
+
+import (
+	"io/ioutil"
+	"net/http"
+	"net/url"
+)
+
+// GetConfig - returns the config.json of a minio setup.
+func (adm *AdminClient) GetConfig() ([]byte, error) {
+	queryVal := make(url.Values)
+	queryVal.Set("config", "")
+
+	hdrs := make(http.Header)
+	hdrs.Set(minioAdminOpHeader, "get")
+
+	reqData := requestData{
+		queryValues:   queryVal,
+		customHeaders: hdrs,
+	}
+
+	// Execute GET on /?lock to list locks.
+	resp, err := adm.executeMethod("GET", reqData)
+
+	defer closeResponse(resp)
+	if err != nil {
+		return nil, err
+	}
+
+	// Return the JSON marshalled bytes to user.
+	return ioutil.ReadAll(resp.Body)
+}