Pre and post Traefik reboot hooks

Provide pre and post reboot hooks for Traefik, that can be used to
remove/add to an external load balancer to prevent requests from being
sent during the reboot.

Works best with the --rolling setting, where each hook is called once
per host.

test/integration/docker/deployer/app/.kamal/hooks/post-traefik-reboot

@@ -0,0 +1,3 @@
+#!/bin/sh
+echo "Rebooted Traefik on ${KAMAL_HOSTS}"
+mkdir -p /tmp/${TEST_ID} && touch /tmp/${TEST_ID}/post-traefik-reboot

test/integration/docker/deployer/app/.kamal/hooks/pre-traefik-reboot

@@ -0,0 +1,3 @@
+#!/bin/sh
+echo "Rebooting Traefik on ${KAMAL_HOSTS}..."
+mkdir -p /tmp/${TEST_ID} && touch /tmp/${TEST_ID}/pre-traefik-reboot