Don't use interpolation if it's not required.

Follow up to #11074, but for the core backend ruby files

Interpolation syntax can be error prone so don't use it when you can just
convert to string.

Prefer `variable.to_s` over `"#{variable}"`

Also, some places we use Array#join which returns a String already:
`"#{rate_detail.errors.full_messages.join(', ')}"`
Becomes:

`rate_detail.errors.full_messages.join(', ')`

Also, some places already created a message string but then wanted to interpolate
it for good measure:

```diff
msg = "Authentication failed for userid #{username}, unable to find user object in #{self.class.proper_name}"
-            _log.warn("#{msg}")
+            _log.warn(msg)
```

rubocop -a --only "Lint/StringConversionInInterpolation,Style/UnneededInterpolation" ...


(transferred from ManageIQ/manageiq@62e4980)

lib/miq_automation_engine/engine/miq_ae_engine.rb

@@ -119,7 +119,7 @@ def self.deliver(*args)
 
       if ws.nil? || ws.root.nil?
         message = "Error delivering #{options[:attrs].inspect} for object [#{object_name}] with state [#{state}] to Automate: Empty Workspace"
-        _log.error("#{message}")
+        _log.error(message)
         return nil
       end
 
@@ -138,12 +138,12 @@ def self.deliver(*args)
           options[:ae_state_previous] = YAML.dump(ws.current_state_info) unless ws.current_state_info.empty?
 
           message = "Requeuing #{options.inspect} for object [#{object_name}] with state [#{options[:state]}] to Automate for delivery in [#{ae_retry_interval}] seconds"
-          _log.info("#{message}")
+          _log.info(message)
           deliver_queue(options, :deliver_on => deliver_on)
         else
           if ae_result.casecmp('error').zero?
             message = "Error delivering #{options[:attrs].inspect} for object [#{object_name}] with state [#{state}] to Automate: #{ws.root['ae_message']}"
-            _log.error("#{message}")
+            _log.error(message)
           end
           MiqAeEvent.process_result(ae_result, automate_attrs) if options[:instance_name].to_s.casecmp('EVENT').zero?
         end
@@ -152,7 +152,7 @@ def self.deliver(*args)
       return ws
     rescue MiqAeException::Error => err
       message = "Error delivering #{automate_attrs.inspect} for object [#{object_name}] with state [#{state}] to Automate: #{err.message}"
-      _log.error("#{message}")
+      _log.error(message)
     ensure
       vmdb_object.after_ae_delivery(ae_result.to_s.downcase) if vmdb_object.respond_to?(:after_ae_delivery)
     end
@@ -212,7 +212,7 @@ def self.create_automation_attribute_name(object)
 
   def self.create_automation_attribute_key(object, attr_name = nil)
     klass_name  = create_automation_attribute_class_name(object)
-    return "#{klass_name}" if automation_attribute_is_array?(klass_name)
+    return klass_name.to_s if automation_attribute_is_array?(klass_name)
     attr_name ||= create_automation_attribute_name(object)
     "#{klass_name}::#{attr_name}"
   end

lib/miq_automation_engine/models/miq_ae_method_yaml.rb

@@ -10,7 +10,7 @@ def initialize(filename = nil)
   def define_instance_variables
     @ae_method_obj['object']['attributes'].each do |k, v|
       instance_variable_set("@#{k}", v)
-      singleton_class.class_eval { attr_accessor "#{k}" }
+      singleton_class.class_eval { attr_accessor k.to_s }
       send("#{k}=", v)
     end
   end

lib/miq_automation_engine/service_models/miq_ae_service_methods.rb

@@ -121,7 +121,7 @@ def self.service_now_eccq_insert(server, username, password, agent, queue, topic
       $log.error hserr.backtrace.join("\n")
       raise
     rescue => err
-      _log.error "#{err}"
+      _log.error err.to_s
       $log.error err.backtrace.join("\n")
       raise
     end

lib/miq_automation_engine/service_models/mixins/miq_ae_service_miq_provision_mixin.rb

@@ -57,7 +57,7 @@ def set_dvs(portgroup, switch = portgroup)
   end
 
   def set_vlan(vlan)
-    set_option(:vlan, ["#{vlan}", "#{vlan}"])
+    set_option(:vlan, [vlan.to_s, vlan.to_s])
   end
 
   def get_folder_paths

spec/miq_ae_object_spec.rb

@@ -63,7 +63,7 @@ def value_match(value, xml_value)
   end
 
   it "#process_args_as_attributes with a hash with an object reference" do
-    result = @miq_obj.process_args_as_attributes("VmOrTemplate::vm" => "#{@vm.id}")
+    result = @miq_obj.process_args_as_attributes("VmOrTemplate::vm" => @vm.id.to_s)
     expect(result["vm_id"]).to eq(@vm.id.to_s)
     expect(result["vm"]).to be_kind_of(MiqAeMethodService::MiqAeServiceVmOrTemplate)
   end

spec/service_methods/miq_ae_service_manageiq-providers-cloud_manager-provision_spec.rb

@@ -35,7 +35,7 @@ module MiqAeServiceManageIQ_Providers_CloudManager_ProvisionSpec
           end
 
           %w(template clone_to_vm).each do |provision_type|
-            it "#{provision_type}" do
+            it provision_type.to_s do
               @miq_provision.update_attributes(:provision_type => provision_type)
               expect(ae_svc_prov.target_type).to eq('vm')
             end