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
1 file changed