Alright so DH was my first ever leather toon, so I had quite the issue with mogging it, especially because Legion's Leather aren't really to my taste (that is to say, they're too ugly!).
So I looked it up and saw Warmongering/Wild Combatant gear was moggable by DHs, only to get to Ashran and see that I can't buy them for some reason Q.Q
Anyway, the workaround I found for this was pretty simple: Start a class trial toon, send 12 marks of honor for it (to buy ensemble), use the garrison HS, run to a near outpost (the one in the garrison won't work!) and fly to ashran (org/sw portals dont work!), from there you buy the ensemble at the vendor and mog it immediatly (note that you have to mog it on your trial toon in order for it to unlock for your DH, it only costs 8 gold though and you start with 10 so its ok!).
There, yey, you now have a complete set of both WIld and Warmongering Coloring of the Hellfire Citadel's mog unlocked for your DH!