Fix cidata user-data ca_certs yaml content

[afbjorklund]

This is failing the cloud-config jsonschema validation, later.

  Error: Cloud config schema errors:
    ca_certs.trusted: None is not of type 'array'

• Generated user-data is failing cloud-config schema validation #2265

---

There are some additional fixes, to be followed up in another PR:

• Cloud config schema deprecations: users.0.uid: Changed in version 22.3. The use of string type is deprecated. Use an integer instead.

-    uid: '1000'
+    uid: 1000

• Error: Cloud config schema errors: users.0: Additional properties are not allowed ('ssh-authorized-keys' was unexpected

-    ssh-authorized-keys:
+    ssh_authorized_keys:

These are either version-dependent (on the runtime cloud-init version, which we don't know just yet)

Or depends on the implementation, where the canonical python implementation does a magic replace.

[afbjorklund]

The initial attempt got a bit confusing, since it was trying to validate the config from the test cases...

The actual user-data in the default template looked like:

ca_certs:
    remove_defaults: false
    trusted: null

Better to fail validation, if the boolean pointer is still nil

[jandubois]

Thanks, LGTM