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
  and emits events for the individual components
* Supporting updates to Stanford playbooks

@jbau and @sefk I can show you the testing dashboard I've been using to confirm this; all of this should be installed to staging.