This commit cleans up areas that configure the image_id and image_name.
It enables the ability to not have to specify an image_id or image_name
when booting from a volume.
It also prevents Terraform from reporting an error when an image name is no
longer able to be resolved from an image ID. This usually happens when the
image has been deleted, but there are still running instances that were based
off of it.
The image_id and image_name parameters no longer immediately take a default
value from the OS_IMAGE_ID and OS_IMAGE_NAME environment variables. If no other
resolution of an image_id or image_name were found, then these variables will
be referenced. This further supports booting from a volume.
Finally, documentation was updated to take into account booting from a volume.
This commit renames flavor_ref to flavor_id and adds the flavor_name
parameter. Users can now specify either a flavor ID or name when launching
instances.
