Read admin_pass from meta_data.json and set root password use cloud-init

meta_data.json as follow: {"admin_pass": "123456", "uuid": "18ac0b2e-106f-45d1-bb8c-39e3016c2740", "availability_zone": "nova", "hostname": "a.novalocal", "launch_index": 0, "name": "A"}, cloud-init read the key-value from it and initialize the guest os using them. But cloud-init doesn't set root password using admin_pass and it's useful and convenient to use root user to login in sometimes.