Added device_type support for users and images

Changed users module to require a device type when creating a user.
Changed cli module to require a device type when creating a user,
and added command to list supported device types.
Changed config schema and example file to use openstack.images
object to store device types and image IDs in key/value format.
Changed setupuser module to use image ID based on device type.
Changed user module and cli clear-vm-info command to set the user's
vm_ip and vm_id to undefined; they must be undefined for the
setupuser module to create a new VM when that user logs in.

Change-Id: I56a8b839410b8e479ceda06f74c88f78be1b1972