Clarify the 'towhat' inclusion validation message on policy creation.

https://bugzilla.redhat.com/show_bug.cgi?id=1663562

- Explicitly set the inclusion whitelist as part of the error message
when creating a policy with an invalid 'towhat' value to clarify the
expectation, particularly when creating via the API.

- Modify supportive spec which tests the validity of the policy record.

app/models/miq_policy.rb

@@ -39,7 +39,8 @@ class MiqPolicy < ApplicationRecord
   validates_presence_of     :name, :description, :guid
   validates_uniqueness_of   :name, :description, :guid
   validates :mode, :inclusion => { :in => %w(compliance control) }
-  validates :towhat, :inclusion => { :in => TOWHAT_APPLIES_TO_CLASSES }
+  validates :towhat, :inclusion => { :in      => TOWHAT_APPLIES_TO_CLASSES,
+                                     :message => "should be one of #{TOWHAT_APPLIES_TO_CLASSES.join(", ")}" }
 
   scope :with_mode,   ->(mode)   { where(:mode => mode) }
   scope :with_towhat, ->(towhat) { where(:towhat => towhat) }

spec/models/miq_policy_spec.rb

@@ -314,7 +314,13 @@
     end
 
     it 'reports invalid towhat' do
-      expect(FactoryBot.build(:miq_policy, :towhat => "BobsYourUncle")).not_to be_valid
+      policy = FactoryBot.build(:miq_policy, :towhat => "BobsYourUncle")
+      towhat_error = "should be one of ContainerGroup, ContainerImage, "\
+                     "ContainerNode, ContainerProject, ContainerReplicator, "\
+                     "ExtManagementSystem, Host, PhysicalServer, Vm"
+
+      expect(policy).not_to be_valid
+      expect(policy.errors.messages).to include(:towhat => [towhat_error])
     end
   end
 end