Isolate Coturn from services in the default Docker network

Most (all?) of our Matrix services are running in the `matrix` network,
so they were safe -- not accessible from Coturn to begin with.

Isolating Coturn into its own network is a security improvement
for people who were starting other services in the default
Docker network. Those services were potentially reachable over the
private Docker network from Coturn.

Discussed in #120 (Github Pull Request)

group_vars/matrix-servers

@@ -90,6 +90,7 @@ matrix_corporal_matrix_registration_shared_secret: "{{ matrix_synapse_registrati
 ######################################################################
 
 matrix_coturn_enabled: true
+
 matrix_coturn_turn_external_ip_address: "{{ ansible_host }}"
 
 ######################################################################