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.

lib/mrsk/cli/app.rb

@@ -13,7 +13,7 @@ class Mrsk::Cli::App < Mrsk::Cli::Base
           execute *MRSK.app.tag_current_as_latest
         end
 
-        on(MRSK.hosts) do |host|
+        on(MRSK.hosts, **MRSK.config.group_strategy) do |host|
           roles = MRSK.roles_on(host)
 
           roles.each do |role|
@@ -39,7 +39,7 @@ class Mrsk::Cli::App < Mrsk::Cli::Base
   desc "start", "Start existing app container on servers"
   def start
     with_lock do
-      on(MRSK.hosts) do |host|
+      on(MRSK.hosts, **MRSK.config.group_strategy) do |host|
         roles = MRSK.roles_on(host)
 
         roles.each do |role|