Allow performing boot & start operations in groups

Adds top-level configuration options for `group_limit` and `group_wait`.
When a `group_limit` is present, we'll perform app boot & start
operations on no more than `group_limit` hosts at a time, optionally
sleeping for `group_wait` seconds after each batch.

We currently only do this batching on boot & start operations (including
when they are part of a deployment). Other commands, like `app stop` or
`app details` still work on all hosts in parallel.

test/cli/app_test.rb

@@ -33,6 +33,16 @@ class CliAppTest < CliTestCase
     Thread.report_on_exception = true
   end
 
+  test "boot uses group strategy when specified" do
+    Mrsk::Cli::App.any_instance.stubs(:on).with("1.1.1.1").twice # acquire & release lock
+    Mrsk::Cli::App.any_instance.stubs(:on).with([ "1.1.1.1" ]) # tag container
+
+    # Strategy is used when booting the containers
+    Mrsk::Cli::App.any_instance.expects(:on).with([ "1.1.1.1" ], in: :groups, limit: 3, wait: 30).with_block_given
+
+    run_command("boot", config: :with_group_strategy)
+  end
+
   test "start" do
     run_command("start").tap do |output|
       assert_match "docker start app-web-999", output
@@ -151,7 +161,7 @@ class CliAppTest < CliTestCase
   end
 
   private
-    def run_command(*command)
-      stdouted { Mrsk::Cli::App.start([*command, "-c", "test/fixtures/deploy_with_accessories.yml", "--hosts", "1.1.1.1"]) }
+    def run_command(*command, config: :with_accessories)
+      stdouted { Mrsk::Cli::App.start([*command, "-c", "test/fixtures/deploy_#{config}.yml", "--hosts", "1.1.1.1"]) }
     end
 end

test/fixtures/deploy_with_group_strategy.yml

@@ -0,0 +1,16 @@
+service: app
+image: dhh/app
+servers:
+  web:
+    - "1.1.1.1"
+    - "1.1.1.2"
+  workers:
+    - "1.1.1.3"
+    - "1.1.1.4"
+
+registry:
+  username: user
+  password: pw
+
+group_limit: 3
+group_wait: 30