[docs]: qemu tutorial doesn't define vendor-data metadata

[holmanb]

Documentation request

In the tutorial, user-data is defined, but metadata and vendor-data are not. We should really include a short description.

The text might say something like:

What is vendor-data?

Clouds sometimes require a way to provide a minimal configuration. Vendor-data is cloud-specific, and allows things like providing user-provided ssh key to the instance, and networking configuration for dynamic cloud environments.

Some clouds also provide network configuration to help the instance handle a dynamic network environment. This isn't typically something that a cloud-init user modifies, unless they are managing their own "cloud" environment.

What is metadata?

This is required, and defines the instance-id. Cloud images are often built from images which have already booted (and therefore previously ran cloud-init). Some method of identifying whether to boot cloud-init as if it is the first time is required. The instance-id is that unique identifier. If instance-id changes, cloud-init will re-run as if it is the first time.