Albergues don't supply towels. Best take a small to pack, lightweight,
quick dry tourist towel, not bath towel. Like this
http://www.seatosummit.com/product/?item=Dry+Lite+Towel+™+&o1=2&o2=2&o3=273-42
Yes, you need your own sleeping bag. Some people say that just a sleeping bag (silk) liner is enough. Never for me. I used the bag and blanket even during a heat wave of over 30*C (all albergues I was in did have blankets, in some had to ask, but they all found them).
Typical accommodation includes a place on bunk bed, mattress, pillow, shower, WC (with paper), sink, mirror on the wall, sink for hand washing clothes (your own soap), clothes line (sometimes too short for everybody), some had sheets and pillow cases, some didn't (or you could by disposable ones for 2 euros). Washing machines + dryers were for fee.
I would take rain poncho. Can't realistically expect that there will be no rain at all in 20 day period.