Validate a CoreOS cloud-config file

The coreos-cloudinit binary is CoreOS-specific, so this can only be done from a CoreOS host:

$ coreos-cloudinit -validate --from-file /tmp/cloud-config

After the system has been instaled, the cloud-config file is written to /var/lib/coreos-install/user_data:

$ sudo cat /var/lib/coreos-install/user_data
#cloud-config

hostname: coreos1
ssh_authorized_keys:
  - ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQCZr...

users:
  - name: andy
    groups:
      - sudo
      - docker
    coreos-ssh-import-github: sublimino
...

Any changes to this file should be validated to prevent loss of access to the system on reboot.

$ coreos-cloudinit -validate --from-file /var/lib/coreos-install/user_data
Checking availability of "local-file"
Fetching user-data from datasource of type "local-file"
line 27: error: found a tab character where an intendation space is expected
line 0: warning: incorrect type for "" (want struct)
$ echo $?
1

After fixing that error, the cloud-config file validates:

$ coreos-cloudinit -validate --from-file /var/lib/coreos-install/user_data
Checking availability of "local-file"
Fetching user-data from datasource of type "local-file"
$ echo $?
0