Enhance SIP device management to also be able to manage IAX devices.

As promised, enhanced the SIP device management to also handle IAX devices.

In config.pp, create a concat instance, to manage the iax.conf file, and add a exec statement, in order to reload the iax config if needed.

Further, updated the config/device.pp, updated a bunch of comments, to reflect that it is now able to also manage iax devices. I only added one additional parameter, to the .pp file and the template, for the IAX handling, the one I currently use and need. There might be more IAX specific parameters, but I thought, whenever someone else might require it, its easy to add later on.

The template file for the iax.conf is taken from the asterisk 11.14.1 version, that I use and where I tested against.