Work around mx-puppet-discord failing with "No relay found" after reboot

Related to https://gitlab.com/mx-puppet/discord/mx-puppet-discord/-/issues/117

Looks like the bridge is too quick to start and fails to initialize
itself by connecting to Synapse. It's mostly observed after a system
reboot, because Synapse (and everything else) is slower to start.

Once mx-puppet-discord fails to initialize itself, a "No relay found"
error will be observed any time you try to relay a Matrix message to
Discord. Relaying messages in the other direction (Discord to Matrix)
also fails.

With this workaround (longer delay on mx-puppet-discord startup), I
observe mx-puppet-discord working well, even after a full reboot.
Of course, a proper fix is preferable, instead of delaying by a magic
number of seconds.

roles/matrix-bridge-mx-puppet-discord/templates/systemd/matrix-mx-puppet-discord.service.j2

@@ -17,7 +17,7 @@ ExecStartPre=-{{ matrix_host_command_sh }} -c '{{ matrix_host_command_docker }}
 ExecStartPre=-{{ matrix_host_command_sh }} -c '{{ matrix_host_command_docker }} rm matrix-mx-puppet-discord 2>/dev/null || true'
 
 # Intentional delay, so that the homeserver (we likely depend on) can manage to start.
-ExecStartPre={{ matrix_host_command_sleep }} 5
+ExecStartPre={{ matrix_host_command_sleep }} 15
 
 ExecStart={{ matrix_host_command_docker }} run --rm --name matrix-mx-puppet-discord \
 			--log-driver=none \