all_ems_in_zone is not a scope yet, so we can't chain 'where'

Fixes a bug introduced in:
ManageIQ#14675

We'll do our condition checks on arrays for now, until we can properly
use scopes.  The scopes will require us to change callers to expect
scopes, which is why we're fixing this bug for now.

app/models/mixins/per_ems_worker_mixin.rb

@@ -18,7 +18,7 @@ def all_ems_in_zone
     end
 
     def all_valid_ems_in_zone
-      all_ems_in_zone.where(:enabled => true).select(&:authentication_status_ok?)
+      all_ems_in_zone.select {|e| e.enabled && e.authentication_status_ok?}
     end
 
     def desired_queue_names

spec/models/mixins/per_ems_worker_mixin_spec.rb

@@ -15,6 +15,14 @@
     expect(@worker_class.queue_name_for_ems(@ems)).to eq(@ems_queue_name)
   end
 
+  it ".all_valid_ems_in_zone" do
+    expect(@worker_class.all_valid_ems_in_zone).to be_empty
+
+    @ems.update(:enabled => true)
+    @ems.authentications.first.validation_successful
+    expect(@worker_class.all_valid_ems_in_zone).to eq([@ems])
+  end
+
   it "#worker_options" do
     expect(@worker_record.worker_options).to eq(:guid => @worker_record.guid, :ems_id => @ems.id)
   end