Storage removal

Datastores can't be removed if there are relations to hosts or vms.
We need provide a way to check whether datastore removal is supported
(no vm or host relations). When there condition is not met we need
to notify a user that relations needs to be removed first.

This PR fixes:
https://bugzilla.redhat.com/1439380

app/controllers/application_controller/ci_processing.rb

@@ -2744,9 +2744,15 @@ def configuration_job_delete
   # Delete all selected or single displayed datastore(s)
   def deletestorages
     assert_privileges("storage_delete")
+    storages = find_checked_ids_with_rbac(Storage)
+    unless Storage.batch_operation_supported?('delete', storages)
+      add_flash(_("Only storage without VMs and Hosts can be removed"), :error)
+      return
+    end
+
     datastores = []
     if %w(show_list storage_list storage_pod_list).include?(@lastaction) || (@lastaction == "show" && @layout != "storage") # showing a list, scan all selected hosts
-      datastores = find_checked_ids_with_rbac(Storage)
+      datastores = storages
       if datastores.empty?
         add_flash(_("No %{model} were selected for %{task}") % {:model => ui_lookup(:tables => "storage"), :task => display_name}, :error)
       end