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.
Should pull the 15 from `whapps_config:get_integer`