Switching to open-isns

Quite a big change, but that's what fate#316901 wanted.
Here, for example, and in many similar methods, we change the interface from 
> @return true

to 
> @return [Boolean] whether the command succeeded

That makes me wonder whether the calling code will accidentally break. So I look it up and see that it did not check the return value before and neither it does now. So it is OK, even if kind of useless, and still undocumented.