gnt-cluster modify: dis/enabling storage types

This patch extends the 'gnt-cluster modify' command to manipulate the list
of enabled storage types. Note that this currenlty does no validation
with respect to whether or not there are instances currently using a storage
type that is being removed from the list.

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