Define matrix_coturn_turn_external_ip_address in the playbook group vars

This is more explicit than hiding it in the role defaults.

People who reuse the roles in their own playbook (and not only) may
incorrectly define `ansible_host` to be a hostname or some local address.

Making it more explicit is more likely to prevent such mistakes.

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 }}"
 
 ######################################################################
 #