nbsample: Don't ignore the supplied random seed

Both the initial version [1] and the _iter_sample version [2] of
nbsample unconditionally set random_seed to None, ignoring the passed
value for the parameter.

[1] a9ccc29f5b30c1c0a68ed8c50430493f462966c1
    Merge pull request #608 from akloster/isample
[2] a24d1750fd2896cd2161da5100da120ff0ca303e
    Change nbsample to use _iter_sample