When you see Summer Stock Austin’s staging of “Springtime for Hitler,” the outrageous signature number from the 1968 film made more outrageous in the 2001 stage musical, that’s what you get. The chorus line of tap-dancing goose-steppers, the showgirls fresh from a Berchtesgaden beer hall, the Busby Berkeley-style twirling human swastika – they provoke guffaws because director-choreographer Ginger Morris (with an assist by guest tap choreographer Scott Thompson) and her young performers serve every inanity with enthusiasm and a flair for the ridiculous.