Fixes #38022 - Do not use undefined setting

Sometimes the production.log is filled with a message: "Setting
remote_execution_workers_pool_size has no definition, please define it
before using"

This message does not appear to be a blocker to any of the existing
functionalities, however; it is misleading while troubleshooting the
issues.

lib/foreman_remote_execution/engine.rb

@@ -28,7 +28,7 @@ class Engine < ::Rails::Engine
 
     initializer 'foreman_remote_execution.require_dynflow', :before => 'foreman_tasks.initialize_dynflow' do |app|
       ForemanTasks.dynflow.require!
-      ForemanTasks.dynflow.config.queues.add(DYNFLOW_QUEUE, :pool_size => Setting['remote_execution_workers_pool_size']) if Setting.table_exists? rescue(false)
+      ForemanTasks.dynflow.config.queues.add(DYNFLOW_QUEUE, :pool_size => Setting['remote_execution_workers_pool_size']) if Setting.find_by :name => 'remote_execution_workers_pool_size' rescue(false)
       ForemanTasks.dynflow.config.eager_load_paths << File.join(ForemanRemoteExecution::Engine.root, 'app/lib/actions')
     end