Upgrade configuration wrt enabled storage types

This implements an online update of the configuration for when a configuration
is loaded that does not yet contain the 'enabled_storage_types' attribute.
Note that this will require more changes as dis/enabling of (shared) file
storage at configure time will be removed.
This patch includes adding a mapping from disk templates to storage types. While
I was on it, I also reordered the disk template and storage type parameters
alphabetically, to make dealing with those easier.
Also, this mentions the configuration change in the NEWS file.

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