[Azure] Optimize VK Setup in ACS/AKS (#85)

* Read ACS Credentials for Azure Authentication

Supprt a new environment variable: ACS_CREDENTIAL_LOCATION
Expect the value to be the ACS credential filepath, which is the
/etc/kubernetes/azure.json file generated on the ACS nodes.

If the ACS_CREDENTIAL_LOCATION is specified and loaded,
create the Azure Authentication class from its values.

If the AZURE_AUTHENTICATION_LOCATION is specified and loaded,
its values will overwrite the value above.

Refactor the ACI provider and ACI client to be able to override the SPN by environment variable

providers/azure/acsCredential.go

@@ -0,0 +1,38 @@
+package azure
+
+import (
+	"encoding/json"
+	"fmt"
+	"io/ioutil"
+	"log"
+)
+
+// AcsCredential represents the credential file for ACS
+type AcsCredential struct {
+	Cloud          string `json:"cloud"`
+	TenantID       string `json:"tenantId"`
+	SubscriptionID string `json:"subscriptionId"`
+	ClientID       string `json:"aadClientId"`
+	ClientSecret   string `json:"aadClientSecret"`
+	ResourceGroup  string `json:"resourceGroup"`
+	Region         string `json:"location"`
+}
+
+// NewAcsCredential returns an AcsCredential struct from file path
+func NewAcsCredential(filepath string) (*AcsCredential, error) {
+	log.Printf("Reading ACS credential file %q", filepath)
+
+	b, err := ioutil.ReadFile(filepath)
+	if err != nil {
+		return nil, fmt.Errorf("Reading ACS credential file %q failed: %v", filepath, err)
+	}
+
+	// Unmarshal the authentication file.
+	var cred AcsCredential
+	if err := json.Unmarshal(b, &cred); err != nil {
+		return nil, err
+	}
+
+	log.Printf("Load ACS credential file %q successfully", filepath)
+	return &cred, nil
+}