Make sure CUDA init called during BTL init

Need to ensure that CUDA init is called during init to match corresponding finalize during closing.
open-mpi/ompi:022612c83ba541678015791fab1a51b7c8ce9fb4
open-mpi/ompi:cbb43d5ac3e5fa96a001097f2b4c680ffef74798

@jsquyres can you review?