KAZOO-3081 - added max_digits parameter to cf_disa configuration

Added max_digits configuration for cf_disa.  Also noticed that PIN collection was hardcoded to always collect 6 digits.  I'm not sure how this was supposed to work if the PIN was longer than 6 digits.  Changed it to look at the length of the configured PIN and collect that many digits.  Hopefully that makes sense.
I wonder if this is a security risk? For instance, I could just enter 1s until the system continues, so now I know I need to enter a 6 digit pin. Much easier to brute force than if I think it could be up to 15 digits (or some max length). Thoughts?