Fix "gnt-instance rename" functionality

Commit 91e0748c (Unify the “--no-ip-check” option) broke the options
variable name for ‘--no-ip-check’ but since we don't have a QA test for
instance rename (only burnin test), this was not caught until Issue 86
was opened.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>