Add new cloud-init-datasources element

This moves cloud-init data source configuration to a general purpose
cloud-init-datasources element that can be used to explicitly configure
the list of cloud-init sources that will be queried on first boot.

cloud-init-nocloud now depends on this new element to configure the
datasource_list while continuing to prep the image for a nocloud first boot.

Change-Id: Ibcc3b86d6ca567a23f89b7a1a36bc713e444ef68
Closes-bug: #1316475
Merge "Add new cloud-init-datasources element"