Stanford-oriented Datadog updates

* Datadog explicitly sets hostname in style Stanford uses (relies on
  /etc/hostname being accurate, requires reboot)
* Enables nginx status check module integration for datadog
* Installs nginx access log processor that counts HTTP response codes
* Updates to lots of other places to make sure that their logs appear
  where we expect
* Has sample code showing how to emit events from log contents which make it
  into the Datadog event stream
* Separation of stage and prod datadog configurations
Merge pull request #445 from edx/jrbl/datadog-loghandler

Stanford-oriented Datadog updates