Add an option to back-up existing Azure VM images

[DixonDs]

It would be great to have an option not to delete the existing managed VM image but rather rename it to some other name. Then in case packer succeeds, remove the old image; otherwise rename it back to the original name.

That would allow safer packer executions when it is better to have older VM images at least, rather than none in case of failure.

[ladar]

See issue #6399.

[ghost]

This issue has been automatically migrated to hashicorp/packer-plugin-azure#21 because it looks like an issue with that plugin. If you believe this is not an issue with the plugin, please reply to hashicorp/packer-plugin-azure#21.