Fix Core Refresher if there is no ems_vmware setting

If there is no prototype.ems_refresh setting currently the Ems Refresh
Core worker will raise an exception.

app/models/miq_ems_refresh_core_worker.rb

@@ -6,7 +6,7 @@ class MiqEmsRefreshCoreWorker < MiqWorker
   self.required_roles = ["ems_inventory"]
 
   def self.has_required_role?
-    return false if Settings.prototype.ems_vmware.update_driven_refresh
+    return false if Settings.prototype.try(:ems_vmware).try(:update_driven_refresh)
     super
   end