Director Bryan Singer and writer/producer Simon Kinberg have spoken at length about this in an interview with Collider. They say (emboldening my own);
“All these movies now exist in the same timeline and certainly the intention at the end of Days of Future Past was that final future we saw was the destination for the characters. So barring another time travel or something else that would upset the timeline, that would be the fate of those characters.”
Simon Kinberg, 2016
That would imply that ignoring further time-meddling, the X-Men films are all now leading to the point we saw with the old cast at the end of X-Men: Days of Future Past.
Director Brian Singer expands on that (again, emboldening my own);
“Time can always be fucked with, we’ve now learned that. We’ve now learned that once you alter time that could be the future, but I don’t believe if you look at all the X-Men movies and Days of Future Past, I don’t believe that’s definitive. I’ll kill any of those characters any day I want. They’re all fair game. Anything can happen. When two things are happening simultaneously in quantum physics it’s what’s called the Super Position and when the Observer finally observes the outcome that’s called the ‘Collapsing of the Super Position’ which is what happened when Wolverine woke up and saw all the happiness. So yes that is the outcome we hope for, that is the outcome we aspire to, and that’s the outcome we are moving towards, but we saw in Days of Future Past another dark world. What says that can’t happen again? What says the awakening of a being that has such power and can acquire the power to destabilize that? So anything is possible. That’s what we’d like to think happens, that’s what Simon would like to think is a good outcome, but to me it’s fair game.”
Brian Singer, 2016
It seems that the writer of the film and the director of the film disagree, at least somewhat, on the certainty the future that we saw at the end of X-Men: Days of Future Past.
Ultimately, I believe that this line from Brian Singer best explains why, despite having seen the future at the end of X-Men: Days of Future Past, we can't be sure that future will actually come to fruition;
"The point is time’s immutability. The idea that time is like a river. You can splash it and mess it up and throw rocks in it and shatter it but it eventually kind of coalesces and this is, again, quantum physics theory. It’s all based in quantum physics."
Brian Singer, 2016
The point he is trying to make is that the future we saw at the end of X-Men: Days of Future Past is just one possibility, not a certainty, and that possibility is made more or less likely by events occurring. As such, for all we know something could happen that would now stop the "happy future" seen by Wolverine at the end of X-Men: Days of Future Past from happening.
So in short - no, the outcome of X-Men: Apocalypse is not already known, despite us seeing the "happy future" in X-Men: Days of Future Past.