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.
Yeah, I had that nagging thought, too.  My thought was that it might be good to take the bigger value of either the pin size or some set limit, I just wasn't sure what the most efficient erlang syntax would be for that.  Something like:

PinSize = max(byte_size(Pin), 10)

where 10 could actually be a configurable number...