Add "image_version" explicitly set to "latest" for all source images

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#### Description

The VM image settings include "imae_publisher", "image_sku", and "image_offer". This should be explicitly set to also use "image_version" of "latest". The option is inconsistently listed, and relying on the implicit "latest" option is confusing.

#### Use Case(s)

The Azure tools use the "latest" image by default, but it's implicit, not explicit. Setting it consistently and explicitly helps personnel ifnd the right place to more specifically designate that value.

#### Potential configuration

See: azure/arm/builder_acc_test.go and examples/rhel.json. The

```
--- a/builder/azure/arm/builder_acc_test.go
+++ b/builder/azure/arm/builder_acc_test.go
@@ -380,6 +380,7 @@ func testBuilderUserDataLinux(userdata string) string {
"image_publisher": "Canonical",
"image_offer": "UbuntuServer",
"image_sku": "16.04-LTS",
+ "image_version": "latest",
"user_data_file": "%s",

"location": "South Central US",
```

#### Potential References

Start by reviewing builder/azure/arm/builder_acc_test.go and examples/rhel.json, then locate the other source-image configurations that list image_publisher, image_offer, and image_sku. Add an explicit image_version of "latest" consistently and update the relevant test or example coverage to confirm the setting is present.

Written by the indexing model from the issue text.