From a generic engineer point of view following term might be best fitting:

> Betriebssicherheit

This term is usually applied to a technical (mechanical) structure in German but could also be applied to software when viewed as a systemic whole.

However there is no good generic term in German language. Your given definition and also [Wikipedia][1] define dependability as a mixture of several attributes which are very broad already for its own. In practice software in general is seldom measured like this really. (It probably should.) I am afraid this is why there is no term yet. For a final translation the  context is important then.


  [1]: https://en.wikipedia.org/wiki/Dependability