The initialization of packer core in JSON also validates that `null` variables were set, except in the case of `packer validate --syntax-only` , but after the refactor to allow to have all commands work with HCL2 and JSON this subtlety was lost.

This refactors the initialisation of the core in order to allow to have `packer validate --syntax-only` not error in case a variable is not set. Since these calls are refactored this works for HCL2 too.

fix #9478

* Update validate command to use FixConfig for checking against known
fixers
* Update validation command flag docs
* Add ConfigFixer method to PackerHandler Interface
* Implement ConfigFixer interface in PackerConfig
* Remove all stdout messaging (i.e calls to c.Ui.Say) in the validate
command. The command will only display hcl.Diagnotic messaging when there is an error or warning.

HCL2 Configs
```
⇶  packer validate docker_centos_shell_provisioner.pkr.hcl

```

JSON Configs
```
⇶  packer validate vmware-iso_ubuntu_minimal/vmware-iso_ubuntu_minimal.json
Error: Failed to prepare build: "vmware-iso"

1 error occurred:
        * Deprecated configuration key: 'iso_checksum_type'. Please call `packer fix`
against your template to update your template to be compatable with the current
version of Packer. Visit https://www.packer.io/docs/commands/fix/ for more
detail.

Warning: Fixable configuration found.
You may need to run `packer fix` to get your build to run correctly.
See debug log for more information.

  map[string]interface{}{
        "builders": []interface{}{
                map[string]interface{}{
                        ... // 3 identical entries
                        "guest_os_type":     string("ubuntu-64"),
                        "http_directory":    string("http"),
-                       "iso_checksum":
string("946a6077af6f5f95a51f82fdc44051c7aa19f9cfc5f737954845a6050543d7c2"),
+                       "iso_checksum":
string("sha256:946a6077af6f5f95a51f82fdc44051c7aa19f9cfc5f737954845a6050543d7c2"),
-                       "iso_checksum_type": string("sha256"),
                        "iso_url":
string("http://old-releases.ubuntu.com/releases/14.04.1/ubuntu-14.04.1-server-amd64.iso"),
                        "shutdown_command":  string("echo 'vagrant' | sudo -S shutdown -P now"),
                        ... // 4 identical entries
                },
        },
  }
```

Move vars from Meta to cla args; this forces us to use it in GetConfigFromJSON which in turn forces us into updating console and validate

Also try to synchronize the completion script