Fix gnt-instance reinstall

Commit 55efe6da "Convert instance
reinstall to multi instance model" actually broke instance reinstall for
single-instance cases. This one-liner fixes it.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>
(cherry picked from commit b6e243ab)

scripts/gnt-instance

@@ -554,7 +554,7 @@ def ReinstallInstance(opts, args):
   else:
     if not opts.force:
       usertext = ("This will reinstall the instance %s and remove"
-                  " all data. Continue?") % instance_name
+                  " all data. Continue?") % inames[0]
       if not AskUser(usertext):
         return 1