Module configuration validation

[Joshua Harlow]

It would be nice if modules could say validate the configuration they will be pulling from yaml before using it.

Say a module needs a configuration with an list in it (for whatever usage) and strings in that list. It'd be nice to have a pre-module stage or have the modules use some external library to validate that there expected/desired/known input is correct. This would make it so that the yaml that is documented is verified and situations where a module expects a list + stirngs in that list but say that list has dictionaries (or other types) in it are invalid before the module actually runs.

Right now alot of those kind of checks don't exist (some do though...)

[James Falcon (falcojr)]

Tracked in Github Issues as https://github.com/canonical/cloud-init/issues/2326