Nova instance config drive Metadata Definition

A nova Juno FFE landed to support setting the
img_config_drive property on images to require images
to be booted with a config drive. The Glance Metadata
Definitions should include this property.

See Nova Blueprint:
https://blueprints.launchpad.net/nova/+spec/config-drive-image-property

Change-Id: Iffc4d4f00f3677ed1fa41233ef6cfbc5c46d8602
Closes-bug: 1367981
Merge "Nova instance config drive Metadata Definition"