* ganeti-extstorage-interface: add examples
 * gnt-instance: document the “ext” template
 * remove a few double spaces

Also link to its design doc in the design-2.7 doc

Signed-off-by: Constantinos Venetsanopoulos <cven@grnet.gr>
Reviewed-by: Guido Trotter <ultrotter@google.com>
Signed-off-by: Guido Trotter <ultrotter@google.com>