allow optionally booting from an already provisioned 'backup' image

[rizumu]

This may or may not be a tremendous speed improvement and is low priorty at the moment. On rackspace booting from a custom image takes just as long as booting from a rackspace image. The difference would be in post boot steps, the server could in theory be mostly provisioned and certain steps could be skipped.

In the provision method, before installing the configurator would be a good point to create a provisioned image to later boot from.