Bug 1506865

1) Sync a Docker Repo
2) Goto Content -> Docker Tags
3) Type anything in the Filters 

Expected:
A list showing the search/auto complete options

Actual:
ISE

"GET https://katello.example.com/katello/api/v2/docker_tags/auto_complete_search?grouped=true&organization_id=1&page=1&paged=true&per_page=20&search=&sort_by=name&sort_order=ASC 500 (Internal Server Error)"

with the server raising a stack trace

<pre>
2017-10-27T03:14:01 91094dbe [app] [I] Started GET "/katello/api/v2/docker_tags/auto_complete_search?grouped=true&organization_id=1&page=1&paged=true&per_page=20&search=&sort_by=name&sort_order=ASC" for ::1 at 2017-10-27 03:14:01 +0000
2017-10-27T03:14:01 91094dbe [app] [I] Processing by Katello::Api::V2::DockerTagsController#auto_complete_search as JSON
2017-10-27T03:14:01 91094dbe [app] [I]   Parameters: {"grouped"=>"true", "organization_id"=>"1", "page"=>"1", "paged"=>"true", "per_page"=>"20", "search"=>"", "sort_by"=>"name", "sort_order"=>"ASC", "api_version"=>"v2"}
2017-10-27T03:14:01 91094dbe [app] [I] Current user: admin (administrator)
2017-10-27T03:14:01 91094dbe [app] [D] Setting current user thread-local variable to admin
2017-10-27T03:14:01 91094dbe [app] [D] Setting current organization thread-local variable to Default Organization
2017-10-27T03:14:02 91094dbe [app] [E] NoMethodError: undefined method `complete_for' for #<Katello::DockerMetaTag::ActiveRecord_Relation:0x007fc769a7d4a0>
 | /home/vagrant/.rvm/gems/ruby-2.2.4/gems/activerecord-4.2.9/lib/active_record/relation/delegation.rb:136:in `method_missing'
 | /home/vagrant/.rvm/gems/ruby-2.2.4/gems/activerecord-4.2.9/lib/active_record/relation/delegation.rb:99:in `method_missing'
 | /home/vagrant/katello/app/controllers/katello/concerns/filtered_auto_complete_search.rb:11:in `auto_complete_search'
 | /home/vagrant/.rvm/gems/ruby-2.2.4/gems/actionpack-4.2.9/lib/action_controller/metal/implicit_render.rb:4:in `send_action'
</pre>