Set ServerTokens=Prod in apache2 config

roles/webserver/templates/apache2/conf.d/security.j2

@@ -28,7 +28,7 @@
 # where Full conveys the most information, and Prod the least.
 #
 #ServerTokens Minimal
-ServerTokens OS
+ServerTokens Prod
 #ServerTokens Full
 
 #