OpInstanceSetParams: Merge {off,on}line_inst parameters

Instead of having two separate parameters, a single boolean parameter is
used. Unfortunately we need a third state to say “no change”, so the
value can be None, True or False (similar to other parameters). There
are no user interface changes.

New QA tests are added, too.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>