Spider doesn't spawn without 3 sign ups

[PowerfulBacon]

Spider infestation doesn't trigger without all slots that it asks for being filled. If only 1 or 2 people sign up, then it should spawn with those players instead of requiring that all 3 slots be filled.

[mrmanlikesbt]

drafted_players_amount should be lowered to 2 or we should use set_drafted_players_amount()