pbx: Add a shared account

Many SIP clients require an account.  Add a simple account with a
username and password of "openstack" that people can plug in to their
client if it doesn't support making calls without it.

Change-Id: I08f73a6b1976de6c0bdd2f93cf7e36083fe8b12d

modules/openstack_project/templates/pbx/asterisk/sip.conf.erb

@@ -30,3 +30,8 @@ trustrpid=yes
 sendrpid=yes
 <% end -%>
 <% end -%>
+
+[openstack]
+type=user
+secret=openstack
+context=public