Allow ClusterTemplate to explicitly specify a driver to use for creating
Clusters.
This is initially sourced from the image property 'magnum_driver', but
may be improved to be specified via client in the future.
Falls back to old driver discovery using (coe, server_type, os) tuple to
keep existing behaviour.
Change-Id: I9e206b589951a02360d3cef0282a9538236ef53b
8a30ad3462