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/configuration.rb

@@ -153,6 +153,15 @@ class Mrsk::Configuration
   end
 
 
+  def group_strategy
+    if group_limit.present?
+      { in: :groups, limit: group_limit, wait: group_wait }
+    else
+      {}
+    end
+  end
+
+
   def audit_broadcast_cmd
     raw_config.audit_broadcast_cmd
   end
@@ -237,4 +246,12 @@ class Mrsk::Configuration
           raise "Can't use commit hash as version, no git repository found in #{Dir.pwd}"
         end
     end
+
+    def group_limit
+      raw_config.group_limit&.to_i
+    end
+
+    def group_wait
+      raw_config.group_wait&.to_i
+    end
 end