Fix issue with orch stacks still having references to deleted ems

Introduced in ManageIQ#288

Fixes https://bugzilla.redhat.com/show_bug.cgi?id=1793112

db/migrate/20181016140921_migrate_orch_stacks_to_have_ownership_concept.rb

@@ -114,7 +114,7 @@ def up
       OrchestrationStack.find_each do |stack|
         user = if stack.service.present?
                  stack.service.tenant_identity
-               elsif !stack.ems_id.nil?
+               elsif stack.ext_management_system
                  stack.ext_management_system.tenant_identity
                else
                  User.super_admin

spec/migrations/20181016140921_migrate_orch_stacks_to_have_ownership_concept_spec.rb

@@ -30,6 +30,18 @@
       expect(stack).to have_attributes(:tenant_id => ext_ms.tenant_id, :evm_owner_id => ext_ms.tenant_identity.id, :miq_group_id => ext_ms.tenant_identity.current_group.id)
     end
 
+    it "sets owner, tenant, and group from the user if the ems was deleted but the stack ems id is still set" do
+      ext_ms = ems.create!(:tenant => tenant)
+      stack = orchestration_stack.create!(:ext_management_system => ext_ms)
+      ext_ms.destroy
+      expect(orchestration_stack.count).to eq(1)
+
+      migrate
+      stack.reload
+
+      expect(stack).to have_attributes(:tenant_id => user.current_tenant.id, :evm_owner_id => user.id, :miq_group_id => user.current_group.id)
+    end
+
     it "sets owner, tenant, and group from the service if the service exists and ems doesn't" do
       svc = service.create!(:tenant_id => tenant.id, :miq_group_id => group.id)
       stack = orchestration_stack.create!(:direct_services => [svc])