Update the default extension

This patch updates the default extension.  This is what a caller would
hit if they called sip:openstack.org (as opposed to
sip:6000@openstack.org), or when they are calling in via the PSTN.  It
asks them to enter a conference number and then hit #.  It will give up
after 3 tries and just hang up.

I moved the 'spam' playback that I originally had to only play if you
enter the digits for spam (7726).

Change-Id: Ia82b9b52766ee191e3fd6f20d4e3b7fbb56f1f5b
Update asterisk documentation

Update the documentation for the Asterisk server.  List the relevant
puppet bits, as well as where to find more documentation (the wiki for
user docs on calling in, and the free to read Asterisk book).

Change-Id: I91e97f5230868fe20c6432c046634b605d78b52c