Make DEFAULT_ENABLED_DISK_TEMPLATES a list

This patch makes the constant DEFAULT_ENABLED_DISK_TEMPLATES
a list instead of a set, because the order of the elements actually matters
as the first one is planned to be the default disk template for instance
creation. Also, this constant is always used like a list and thus providing it
as a set complicates the code that uses it.

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