Add pre and post app boot hooks

Add two new hooks pre-app-boot and post-app-boot. They are analagous
to the pre/post proxy reboot hooks.

If the boot strategy deploys in groups, then the hooks are called once
per group of hosts and `KAMAL_HOSTS` contains a comma delimited list of
the hosts in that group.

If all hosts are deployed to at once, then they are called once with
`KAMAL_HOSTS` containing all the hosts.

It is possible to have pauses between groups of hosts in the boot config,
where this is the case the pause happens after the post-app-boot hook is
called.

lib/kamal/cli/app.rb

@@ -16,10 +16,18 @@ class Kamal::Cli::App < Kamal::Cli::Base
         # Primary hosts and roles are returned first, so they can open the barrier
         barrier = Kamal::Cli::Healthcheck::Barrier.new
 
-        on(KAMAL.hosts, **KAMAL.boot_strategy) do |host|
-          KAMAL.roles_on(host).each do |role|
-            Kamal::Cli::App::Boot.new(host, role, self, version, barrier).run
+        host_boot_groups.each do |hosts|
+          host_list = Array(hosts).join(",")
+          run_hook "pre-app-boot", hosts: host_list
+
+          on(hosts) do |host|
+            KAMAL.roles_on(host).each do |role|
+              Kamal::Cli::App::Boot.new(host, role, self, version, barrier).run
+            end
           end
+
+          run_hook "post-app-boot", hosts: host_list
+          sleep KAMAL.config.boot.wait if KAMAL.config.boot.wait
         end
 
         # Tag once the app booted on all hosts
@@ -340,4 +348,8 @@ class Kamal::Cli::App < Kamal::Cli::Base
         yield
       end
     end
+
+    def host_boot_groups
+      KAMAL.config.boot.limit ? KAMAL.hosts.each_slice(KAMAL.config.boot.limit).to_a : [ KAMAL.hosts ]
+    end
 end

lib/kamal/cli/templates/sample_hooks/post-app-boot.sample

@@ -0,0 +1,3 @@
+#!/bin/sh
+
+echo "Booted app version $KAMAL_VERSION on $KAMAL_HOSTS..."

lib/kamal/cli/templates/sample_hooks/pre-app-boot.sample

@@ -0,0 +1,3 @@
+#!/bin/sh
+
+echo "Booting app version $KAMAL_VERSION on $KAMAL_HOSTS..."

lib/kamal/commander.rb

@@ -136,14 +136,6 @@ class Kamal::Commander
     SSHKit.config.output_verbosity = old_level
   end
 
-  def boot_strategy
-    if config.boot.limit.present?
-      { in: :groups, limit: config.boot.limit, wait: config.boot.wait }
-    else
-      {}
-    end
-  end
-
   def holding_lock?
     self.holding_lock
   end