Add KAMAL.app_hosts

KAMAL.hosts includes accessory and apps hosts. Add KAMAL.app_hosts which
does not include accessory only hosts and use it for app specific
commands.

Fixes:
- https://github.com/basecamp/kamal/issues/1059
- https://github.com/basecamp/kamal/issues/1148

test/integration/docker/deployer/app/config/deploy.yml

@@ -41,3 +41,8 @@ accessories:
     cmd: sh -c 'echo "Starting busybox..."; trap exit term; while true; do sleep 1; done'
     roles:
       - web
+  busybox2:
+    service: custom-busybox
+    image: registry:4443/busybox:1.36.0
+    cmd: sh -c 'echo "Starting busybox..."; trap exit term; while true; do sleep 1; done'
+    host: vm3