FS: Bind external IP by interface

Let's use freeswitch's interface: capability rather than specify a static IP
to bind to, which breaks then on systems attached to a DHCP router.

modules/rhizo_base/manifests/freeswitch.pp

@@ -109,6 +109,7 @@ class rhizo_base::freeswitch::common {
 
   $vpn_ip_address = $rhizo_base::vpn_ip_address
   $wan_ip_address = $rhizo_base::wan_ip_address
+  $fs_bound_if    = $rhizo_base::stats_if
 
   $voip_username  = $rhizo_base::voip_username
   $voip_fromuser  = $rhizo_base::voip_fromuser