roles/coturn: use only loopback topology

To avoid excessive consumption of UDP port that seems to be detected
and blocked by SIAF upstream firewall.

Only loopback relay is permitted, so the only TURN Topology allowed
is:

Forward:
Client 1 <--TURN--> (udp/88) Server <--RTP--> Server (udp/88) <--TURN--> Client 2

Topologies like:

Client 1 <--TURN--> (udp/88) Server (udp/>10000) <--RTP--> Client 2

Are not allowed.

roles/coturn/templates/turnserver.conf.j2

@@ -8,7 +8,7 @@ external-ip={{ public_ip }}
 min-port={{ min_relay_port }}
 max-port={{ max_relay_port }}
 
-#fingerprint
+fingerprint
 #lt-cred-mech
 
 use-auth-secret
@@ -28,7 +28,9 @@ total-quota=1200
 #no-udp-relay
 no-tcp-relay
 
-denied-peer-ip=10.0.0.0-10.255.255.255
+denied-peer-ip=0.0.0.0-255.255.255.255
+allowed-peer-ip={{ public_ip }}
+allowed-peer-ip={{ ansible_host }}
 
 no-tlsv1
 no-tlsv1_1