Fix gnt-cluster modify check of passed options

… and add the disk_templates/vcpu_ratio new entries. Since the default
ispecs_* are dicts, we can't compare them to none.

Additionally, add a QA test for this case (empty cluster modify).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>