Remove hardcoded command paths in systemd unit files

Depending on the distro, common commands like sleep and chown may either
be located in /bin or /usr/bin.

Systemd added path lookup to ExecStart in v239, allowing only the
command name to be put in unit files and not the full path as
historically required. At least Ubuntu 18.04 LTS is however still on
v237 so we should maintain portability for a while longer.

roles/matrix-coturn/templates/systemd/matrix-coturn.service.j2

@@ -8,10 +8,10 @@ After={{ service }}
 
 [Service]
 Type=simple
-ExecStartPre=-/usr/bin/docker kill matrix-coturn
-ExecStartPre=-/usr/bin/docker rm matrix-coturn
+ExecStartPre=-{{ matrix_host_command_docker }} kill matrix-coturn
+ExecStartPre=-{{ matrix_host_command_docker }} rm matrix-coturn
 
-ExecStart=/usr/bin/docker run --rm --name matrix-coturn \
+ExecStart={{ matrix_host_command_docker }} run --rm --name matrix-coturn \
 			--log-driver=none \
 			--user={{ matrix_user_uid }}:{{ matrix_user_gid }} \
 			--cap-drop=ALL \
@@ -40,12 +40,12 @@ ExecStart=/usr/bin/docker run --rm --name matrix-coturn \
 			{{ matrix_coturn_docker_image }} \
 			-c /turnserver.conf
 
-ExecStop=-/usr/bin/docker kill matrix-coturn
-ExecStop=-/usr/bin/docker rm matrix-coturn
+ExecStop=-{{ matrix_host_command_docker }} kill matrix-coturn
+ExecStop=-{{ matrix_host_command_docker }} rm matrix-coturn
 
 # This only reloads certificates (not other configuration).
 # See: https://github.com/coturn/coturn/pull/236
-ExecReload=/usr/bin/docker exec matrix-coturn kill -USR2 1
+ExecReload={{ matrix_host_command_docker }} exec matrix-coturn kill -USR2 1
 
 Restart=always
 RestartSec=30