Add a simple implementation of element dependency

This adds a new optional file to the root of elements. The
file lists dependencies which will be added to the list
requested by the user during disk image creation.

Change-Id: Id71c3b333563604bbbaf90f9cf40e24fa9738fc8
Enable 'ec2-user' in HEAT for quantal images.

Employs some workarounds that can be removed when bugs reference in
comments are fixed elsewhere.

Change-Id: If423844f0dfabbddd4d0d7b8ad8fcf6d32693195