Bump zuul v3 executor disk limit

We need to work out what the real value of this is, but for the
migration open it fairly wide.

Change-Id: I3486d5a3f398a228d204c7987acd44967a9e8d20

manifests/site.pp

@@ -1170,7 +1170,7 @@ node /^ze\d+\.openstack\.org$/ {
     # properly. We need to revisting this post Queens PTG.
     trusted_ro_paths         => ['/etc/openafs', '/var/lib/zuul/ssh'],
     trusted_rw_paths         => ['/afs'],
-    disk_limit_per_job       => 500,  # Megabytes
+    disk_limit_per_job       => 5000,  # Megabytes
     site_variables_yaml_file => $::project_config::zuul_site_variables_yaml,
     require                  => $::project_config::config_dir,
   }