Check for agent class file to determine if it's valid (#1907)

The previous approach of checking if the class is defined caused problems when
the agent class had not yet been loaded by the process. Using Agent.valid_type?
instead seems to be the approach that is taken elsewhere in the codebase, and it
does not suffer from this problem.

app/models/agent.rb

@@ -418,7 +418,7 @@ class Agent < ActiveRecord::Base
       return if schedule == 'never'
       types = where(:schedule => schedule).group(:type).pluck(:type)
       types.each do |type|
-        next unless const_defined?(type)
+        next unless valid_type?(type)
         type.constantize.bulk_check(schedule)
       end
     end