Check if instances use disk templates that get disabled

When disk templates get disabled via 'gnt-cluster modify', so far
we did not check if any instances still use a disk template of that
type. This patch adds the necessariy check.
It also includes adding a map of disk templates to storage types
and reordering of storage types and disk templates in alphabetical order.

Signed-off-by: Helga Velroyen <helgav@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>