acloud: Set default machine_type to n1-standard-4.

Bug: 113501753
Test: m acloud && ran below create cmd to check machineType.
acloud create_cf --branch git_master \
--build_target cf_x86_phone-userdebug \
--build_id 4978353 \
--config_file ~/acloud.config \
-vv

Change-Id: I55a6b816cb82c81f411ecd794061a9c0bc16ef03
diff --git a/public/data/default.config b/public/data/default.config
index 88ee324..f81c756 100644
--- a/public/data/default.config
+++ b/public/data/default.config
@@ -1,4 +1,4 @@
-min_machine_size: "n1-standard-1"
+min_machine_size: "n1-standard-4"
 disk_image_name: "avd-system.tar.gz"
 disk_image_mime_type: "application/x-tar"
 disk_image_extension: ".tar.gz"
@@ -9,7 +9,7 @@
 user_agent: "acloud"
 
 default_usr_cfg {
-  machine_type: "n1-standard-1"
+  machine_type: "n1-standard-4"
   network: "default"
   extra_data_disk_size_gb: 0