gnt-instance rename: Fix bug and rename params

This patch fixes a bug when gnt-instance rename was invoked with
--no-name-check. It renames the internal variables to be consistent with
the ones in equivalent instance add code. Furthermore it checks whether
and instance rename is invoked with --no-name-check but without
--no-ip-check and throws an exception if so.

Signed-off-by: Manuel Franceschini <livewire@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>