Fixes #23662 - fixes not null insert on content-repo

association.  Rails 4.2 changed the default timestamp creation from
null => true to null => false.  This corrects these tables

db/migrate/20180521200848_drop_repo_rpms_timestamp_not_null.rb

@@ -0,0 +1,18 @@
+class DropRepoRpmsTimestampNotNull < ActiveRecord::Migration[5.1]
+  def change
+    tables = [:katello_repository_docker_manifest_lists,
+              :katello_repository_docker_manifests,
+              :katello_repository_errata,
+              :katello_repository_files,
+              :katello_repository_ostree_branches,
+              :katello_repository_package_groups,
+              :katello_repository_puppet_modules,
+              :katello_repository_rpms,
+              :katello_repository_srpms]
+
+    tables.each do |table|
+      change_column table, :created_at, :datetime, :null => true
+      change_column table, :updated_at, :datetime, :null => true
+    end
+  end
+end