thumbnails inherit their s3 bucket from their attachment

test plan:
 * enable s3 storage
 * configure plugin settings to a different bucket than s3.yml
 * ensure that avatar thumbnails are uploaded and retrieved from
   the different bucket

Change-Id: I7fbf09fa8423c7a916d3b0a602606e2370b7837c
Reviewed-on: https://gerrit.instructure.com/10942
Tested-by: Cody Cutrer <cody@instructure.com>
Reviewed-by: Brian Palmer <brianp@instructure.com>

app/models/thumbnail.rb

@@ -40,6 +40,10 @@ class Thumbnail < ActiveRecord::Base
                    :keep_profile => true
   end
 
+  def bucket_name
+    self.attachment.bucket_name
+  end
+
   def cached_s3_url
     @cached_s3_url = authenticated_s3_url(:expires_in => 144.hours)
   end

spec/models/attachment_spec.rb

@@ -770,6 +770,11 @@ describe Attachment do
       Attachment.domain_namespace = nil
       attachment_model
       @attachment.s3_config[:bucket_name].should == 'pluginsetting_bucket'
+      # if local storage is configured, this will return "no-bucket"
+      @attachment.stubs(:bucket_name).returns('pluginsetting_bucket')
+
+      # thumbnails should use the same bucket as the attachment they are parented to
+      Thumbnail.new(:attachment => @attachment).bucket_name.should == 'pluginsetting_bucket'
     end
   end
 end