Netplan validate can be used to check the configuration before trying to
apply it. It's useful when one is making changes in the configuration
and want to periodically check if it is still valid without having to call
"apply" or "generate".

The validate command will load, parse and validate the configuration
without saving anything to disk.

Due to the change in netplan.cli.core (which prevents it of raising
exception (which were not being handled and resulting in crashes)), a
small change was made in the cli tests so the main() method is not
called directly anymore.

Add the new "netplan validate" command to it.

--debug will show what files are being processed and which ones were
shadowed.