Add robots.txt to gerrit.

And slow down bing (msnbot).

Change-Id: Id8361047abc2cfb52260b3d0ef01275ec3a923f5
Reviewed-on: https://review.openstack.org/32435
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Elizabeth Krumbach Joseph <lyz@princessleia.com>
Reviewed-by: Anita Kuno <anita.kuno@enovance.com>
Approved: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins