Add missing tests for commit f0d22861

Commit f0d22861 changed the logic of
gnt_instance._ConvertNicDiskModifications to also allow a parameter
named “modify”. Unfortunately the corresponding unittest was not
updated. An “if”/“else” condition is also merged.

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