limit facebook user searching to pertinent shards

closes CNVS-3834

test plan:
 * facebook app should still work

Change-Id: Ifd892a0b00afa173ec1d9d1f973955c3fbb806a1
Reviewed-on: https://gerrit.instructure.com/17662
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jacob Fugal <jacob@instructure.com>
QA-Review: Clare Hetherington <clare@instructure.com>

app/controllers/facebook_controller.rb

@@ -109,7 +109,7 @@ class FacebookController < ApplicationController
       data, sig = Facebook.parse_signed_request(params[:signed_request])
       if data && sig
         if @facebook_user_id = data['user_id']
-          Shard.with_each_shard do
+          Shard.with_each_shard(UserService.associated_shards('facebook', @facebook_user_id)) do
             @service = UserService.find_by_service_and_service_user_id('facebook', @facebook_user_id)
             break if @service
           end
@@ -130,7 +130,7 @@ class FacebookController < ApplicationController
       @service = @user.user_services.find_by_service('facebook')
     elsif session[:facebook_user_id]
       @facebook_user_id = session[:facebook_user_id]
-      Shard.with_each_shard do
+      Shard.with_each_shard(UserService.associated_shards('facebook', @facebook_user_id)) do
         @service = UserService.find_by_service_and_service_user_id('facebook', @facebook_user_id)
         break if @service
       end

app/models/user_service.rb

@@ -280,4 +280,8 @@ class UserService < ActiveRecord::Base
     end
   end
   def self.serialization_excludes; [:crypted_password, :password_salt, :token, :secret]; end
+
+  def self.associated_shards(service, service_user_id)
+    [Shard.default]
+  end
 end