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-startup-msg.go

@@ -0,0 +1,67 @@
+/*
+ * Minio Cloud Storage, (C) 2016, 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 (
+	"fmt"
+	"runtime"
+	"strings"
+
+	"github.com/minio/mc/pkg/console"
+)
+
+// Prints the formatted startup message.
+func printGatewayStartupMessage(apiEndPoints []string, accessKey, secretKey, backendType string) {
+	// Prints credential.
+	printGatewayCommonMsg(apiEndPoints, accessKey, secretKey)
+
+	// Prints `mc` cli configuration message chooses
+	// first endpoint as default.
+	endPoint := apiEndPoints[0]
+
+	// Configure 'mc', following block prints platform specific information for minio client.
+	console.Println(colorBlue("\nCommand-line Access: ") + mcQuickStartGuide)
+	if runtime.GOOS == globalWindowsOSName {
+		mcMessage := fmt.Sprintf("$ mc.exe config host add my%s %s %s %s", backendType, endPoint, accessKey, secretKey)
+		console.Println(fmt.Sprintf(getFormatStr(len(mcMessage), 3), mcMessage))
+	} else {
+		mcMessage := fmt.Sprintf("$ mc config host add my%s %s %s %s", backendType, endPoint, accessKey, secretKey)
+		console.Println(fmt.Sprintf(getFormatStr(len(mcMessage), 3), mcMessage))
+	}
+
+	// Prints documentation message.
+	printObjectAPIMsg()
+
+	// SSL is configured reads certification chain, prints
+	// authority and expiry.
+	if globalIsSSL {
+		certs, err := readCertificateChain()
+		if err != nil {
+			console.Fatalf("Unable to read certificate chain. Error: %s", err)
+		}
+		printCertificateMsg(certs)
+	}
+}
+
+// Prints common server startup message. Prints credential, region and browser access.
+func printGatewayCommonMsg(apiEndpoints []string, accessKey, secretKey string) {
+	apiEndpointStr := strings.Join(apiEndpoints, "  ")
+	// Colorize the message and print.
+	console.Println(colorBlue("\nEndpoint: ") + colorBold(fmt.Sprintf(getFormatStr(len(apiEndpointStr), 1), apiEndpointStr)))
+	console.Println(colorBlue("AccessKey: ") + colorBold(fmt.Sprintf("%s ", accessKey)))
+	console.Println(colorBlue("SecretKey: ") + colorBold(fmt.Sprintf("%s ", secretKey)))
+}