A more general fix for selects in NLG preprocessing

If there is a number suffix in the slot ID, it gets removed
before performing preprocessing, so that the same actions
are applied to `vehicle1` and `vehicle` etc.
Slot ID = slot name + (optional) number to distinguish different values for the same slot used in a single NLG template.

Since there are currently two usages of the same slot at most (when asking the user to select between two values), I think it's OK to use a single digit.