Allow matrix_coturn_docker_network to be set to 'host' to use host-networking

This helps large deployments which need to open up thousands of ports
(matrix_coturn_turn_udp_min_port, matrix_coturn_turn_udp_min_port)

On a test VM, opening 1k ports takes 17 seconds for Docker to "publish"
all of these ports (setting up forwarding rules with the firewall, etc),
so service startup and shutdown take a long amount of time.

If host-networking is used, there's no need to open any ports at all
and startup/shutdown can be quick.

roles/custom/matrix-coturn/tasks/setup_install.yml

@@ -62,7 +62,8 @@
     owner: "{{ matrix_user_username }}"
     group: "{{ matrix_user_groupname }}"
 
-- name: Ensure Coturn network is created in Docker
+- when: matrix_coturn_docker_network not in ['', 'host']
+  name: Ensure Coturn network is created in Docker
   community.docker.docker_network:
     name: "{{ matrix_coturn_docker_network }}"
     driver: bridge