nagios: Add configuration designed for full-stack servers.

This doesn't yet pass all Nagios checks correctly, and still has a few
flaws:
* The ideal setup code for the `nagios` user in the database isn't included.
* Some of the other details are a bit off; we need to split some host roles.

But it's better than nothing, and we can iterate from here.

puppet/zulip_ops/manifests/nagios.pp

@@ -29,6 +29,7 @@ class zulip_ops::nagios {
   $hosts_redis = split(zulipconf("nagios", "hosts_redis", undef), ",")
   $hosts_loadbalancer = split(zulipconf("nagios", "hosts_loadbalancer", undef), ",")
   $hosts_stats = split(zulipconf("nagios", "hosts_stats", undef), ",")
+  $hosts_fullstack = split(zulipconf("nagios", "hosts_fullstack", undef), ",")
 
   apache2site { 'nagios':
     require => [File['/etc/apache2/sites-available/'],

puppet/zulip_ops/templates/nagios3/hosts.cfg.template.erb

@@ -18,6 +18,16 @@ define host{
        }
 <% end -%>
 
+<% @hosts_fullstack.each do |host| -%>
+define host{
+       use              generic-host
+       host_name        <%= host %>
+       alias            <%= host %>
+       address          <%= host %>
+       hostgroups       all,web,frontends,staging_frontends,not_pageable_servers,postgres,postgres_appdb_primary
+       }
+<% end -%>
+
 <% @hosts_app_staging.each do |host| -%>
 define host{
        use              generic-host