Can a ranger that has an optional feature to use druidic focus also use a staff that requires a druid to be attuned to (I my opinion that means it's a druidic focus) for example Staff of the Woodlands?

By RAW, no. If they could attune to it, it would say so in the description. 

Yeah, I think you could use that staff as a druidic focus, but you couldn't attune to it, because you're not a Druid.

If you're really excited about the staff, multiclassing with a level of Druid isn't a totally insane idea for a Ranger.