make account reports only run once and low priority

fixes CNVS-8212

test plan 
 - run an account report
 - report should work

Change-Id: I38b1457d5e1428101273f640f62696ec13dc0d39
Reviewed-on: https://gerrit.instructure.com/24427
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>
Product-Review: Rob Orton <rob@instructure.com>

app/models/account_report.rb

@@ -68,7 +68,7 @@ class AccountReport < ActiveRecord::Base
       self.save
     end
   end
-  handle_asynchronously :run_report
+  handle_asynchronously :run_report, :priority => Delayed::LOW_PRIORITY, :max_attempts => 1
 
   def has_parameter?(key)
     self.parameters.is_a?(Hash) && self.parameters[key].presence