Set zuulv3 executor disk limit to 500MB

Based on 150 jobs per executor and 80G for both the merger and
jobdirs, that's *slightly* oversubscribed, but I believe we should
be able to oversubscribe a bit.

Change-Id: I9e9d647fddab356479e26cc5400042d547bec45a
Depends-On: Ic267a156ee7eef4eb9e5f431cc22b236c017d0e8

manifests/site.pp

@@ -1100,6 +1100,7 @@ node /^ze\d+\.openstack\.org$/ {
     gearman_client_ssl_key  => hiera('gearman_client_ssl_key'),
     gearman_ssl_ca          => hiera('gearman_ssl_ca'),
     trusted_ro_dirs         => ['/var/lib/zuul/ssh'],
+    disk_limit_per_job      => 500,  # Megabytes
   }
 
   class { '::zuul::executor': }