docs: mention platform naming skew

[lucab]

Afterburn usually follows Ignition platform naming convention, in order to auto-detect the platform.

However there are several platforms where the metadata component is only optional. On those platforms, in case of failures, Afterburn cannot distinguish a known-good state (i.e. this infra lacks the metadata component) from a failed state (i.e. network or logic error).

As such, Afterburn knows about the following platform which are more specific than the Ignition ones, cannot be auto-detected, and require user-provided hints (as cmdline flags):

• cloudstack-metadata
• cloudstack-configdrive
• openstack-metadata
• vagrant-virtualbox

[lucab]

This follows up to @cgwalters's doubt from https://github.com/openshift/machine-config-operator/pull/964#issuecomment-511036266.

[bgilbert]

openstack-metadata has since been consolidated into a platform ID openstack that does match Ignition. vagrant-virtualbox has been removed, and https://github.com/coreos/afterburn/issues/856 tracks consolidation of the CloudStack providers into a cloudstack one. Closing this in favor of fixing CloudStack to use the Ignition platform ID.