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-synapse/templates/synapse/systemd/matrix-synapse.service.j2

@@ -11,16 +11,16 @@ Wants={{ service }}
 
 [Service]
 Type=simple
-ExecStartPre=-/usr/bin/docker kill matrix-synapse
-ExecStartPre=-/usr/bin/docker rm matrix-synapse
+ExecStartPre=-{{ matrix_host_command_docker }} kill matrix-synapse
+ExecStartPre=-{{ matrix_host_command_docker }} rm matrix-synapse
 {% if matrix_s3_media_store_enabled %}
 # Allow for some time before starting, so that media store can mount.
 # Mounting can happen later too, but if we start writing,
 # we'd write files to the local filesystem and fusermount will complain.
-ExecStartPre=/bin/sleep 3
+ExecStartPre={{ matrix_host_command_sleep }} 3
 {% endif %}
 
-ExecStart=/usr/bin/docker run --rm --name matrix-synapse \
+ExecStart={{ matrix_host_command_docker }} run --rm --name matrix-synapse \
 			--log-driver=none \
 			--user={{ matrix_user_uid }}:{{ matrix_user_gid }} \
 			--cap-drop=ALL \
@@ -55,9 +55,9 @@ ExecStart=/usr/bin/docker run --rm --name matrix-synapse \
 			{{ matrix_synapse_docker_image }} \
 			-m synapse.app.homeserver -c /data/homeserver.yaml
 
-ExecStop=-/usr/bin/docker kill matrix-synapse
-ExecStop=-/usr/bin/docker rm matrix-synapse
-ExecReload=/usr/bin/docker exec matrix-synapse kill -HUP 1
+ExecStop=-{{ matrix_host_command_docker }} kill matrix-synapse
+ExecStop=-{{ matrix_host_command_docker }} rm matrix-synapse
+ExecReload={{ matrix_host_command_docker }} exec matrix-synapse kill -HUP 1
 Restart=always
 RestartSec=30
 SyslogIdentifier=matrix-synapse