Add another 1.1->1.2 compatibility alias

gnt-instance replace-disks used to be called replace_disks.

Reviewed-by: iustinp

scripts/gnt-instance

@@ -947,6 +947,7 @@ commands = {
 
 aliases = {
   'activate_block_devs': 'activate-disks',
+  'replace_disks': 'replace-disks',
   'start': 'startup',
   'stop': 'shutdown',
   }