[ACI] Support to add extra user agent to the request (#378)

* Add UserAgent

* Change to join the user agent with a whitespace and set in the header

* Remove empty entry from user agent

providers/azure/client/aci/client_test.go

@@ -39,7 +39,7 @@ func TestMain(m *testing.M) {
 	subscriptionID = auth.SubscriptionID
 
 	// Check if the resource group exists and create it if not.
-	rgCli, err := resourcegroups.NewClient(auth)
+	rgCli, err := resourcegroups.NewClient(auth, "unit-test")
 	if err != nil {
 		log.Fatalf("creating new resourcegroups client failed: %v", err)
 	}
@@ -82,7 +82,7 @@ func TestNewClient(t *testing.T) {
 		log.Fatalf("Failed to load Azure authentication file: %v", err)
 	}
 
-	c, err := NewClient(auth)
+	c, err := NewClient(auth, "unit-test")
 	if err != nil {
 		t.Fatal(err)
 	}