returning var_list to Optimizer in order
returning var_list to Optimizer in order

- Issue appears to affect only ScipyOptimizer. Does not affect, e.g.,
  AdamOptimizer on doc/source/notebook/classification
- Appears to be due to the fact that TensorFlow variables are not stably
  ordered across across replications, so var_list (and thus random
initial values for optimization) is not either.
GPflow/GPflow