Cloud-init inconsistently uses config

[Brett Holman]

On boot, cloud-init is able to pass cloud-config via userdata_raw, a useful feature when testing behavior on a datasource that doesn't have its own way of retrieving userdata.

system_info:
   datasource:
     None:
       userdata_raw: "#cloud-config\ngrub_dpkg:\n enabled: true"

This works during boot as one might expect:

2023-03-03 21:15:59,255 - modules.py[DEBUG]: Running module grub-dpkg (<module 'cloudinit.config.cc_grub_dpkg' from '/
usr/lib/python3/dist-packages/cloudinit/config/cc_grub_dpkg.py'>) with frequency once-per-instance
2023-03-03 21:15:59,256 - handlers.py[DEBUG]: start: modules-config/config-grub-dpkg: running config-grub-dpkg with frequency once-per-instance

However, cloud-init's single subcommand clearly uses a different configuration, since the following fails to run the same module with the same config:

```
cloud-init --debug --force single --frequency always --name cc_grub_dpkg
```

Furthermore, this userdata_raw isn't included in /run/cloud-init/instance-data-sensitive.json, nor in cloud-init query -a.

Alarmingly, an invalid config provided via userdata_raw is not warned of by cloud-init schema --system:

```
system_info:
   # This will affect which distro class gets used
   datasource:
     NoCloud:
       userdata_raw: |
         #cloud-config
         grub_dpkg:
           enabled: true
           invalid-key: true
```

output:
```
# cloud-init schema --system
Found cloud-config data types: user-data, vendor-data

1. user-data at /var/lib/cloud/instances/cloudinit-0302-160255pw859u9h/cloud-config.txt:
  Valid cloud-config: user-data

2. vendor-data at /var/lib/cloud/instances/cloudinit-0302-160255pw859u9h/vendor-cloud-config.txt:
  Valid cloud-config: vendor-data
 ```

These details reveal inconsistency in cloud-init config handling, and all contribute to a confusing user experience when using userdata_raw.

There may be other inconsistencies too - configs passed directly on the kernel commandline (cc: <config> end_cc), and configs sourced from a url via the kernel commandline (cloud-config-url=) are both used during boot - but I haven't looked to see whether they behave correctly with (schema|single|query) subcommands or show up in /run/cloud-init/instance-data-sensitive.json, but after this finding we probably aught to audit for other issues like this.

[James Falcon (falcojr)]

Tracked in Github Issues as https://github.com/canonical/cloud-init/issues/4086