Move group_limit & group_wait under boot

Also make formatting the group strategy the responsibility of the
commander.

lib/mrsk/configuration/boot.rb

@@ -0,0 +1,9 @@
+class Mrsk::Configuration::Boot
+  attr_reader :group_wait, :group_limit
+
+  def initialize(section:)
+    section = section || {}
+    @group_limit = section["group_limit"]
+    @group_wait = section["group_wait"]
+  end
+end