Install Asterisk sounds

In addition to installing Asterisk itself, we want to install some sound
packages.  This includes a large set of prompts, as well as some music on
hold files.
Set up basic Asterisk configuration

This commit sets up the basic configuration for Asterisk.  It will allow
Asterisk to run, but it won't do anything useful yet.

Change-Id: I7975082ff5351db4dc6e3c8cf9dd2d90675e3108