Set up monitoring alerts on the staging server.

This will fix #720 and #518.

- [x] Change the notification plan at deployment-time to send email to a configurable address, instead.
- [x] Allow `monitoring.yml` to be executed independently of `site.yml` by collecting its own facts about the relevant load balancer.
- [x] Install the same monitoring setup on the staging load balancer, too.
- [x] Figure out how to add the monitoring endpoints to the IP access list so the checks can actually work.
- [x] Use the staging key in PagerDuty for notifications on staging.
- [x] Fix the local `rax_clb` module to not unapply SSL settings when `ssl_termination` or `https_redirect` are omitted.
- [x] Use `set_facts` instead of `register`/`when` to choose the active notification plan.