Run subunit2sql migrations in the background

This commit switches to running migrations in the background because
of timeout issues. As part of this it also adds an output file to
/var/log to store the output of the commands so it's easy to debug
if something goes wrong.

Change-Id: Id702df88d7eeb06bdb727a178bad232c9fa0d1f8

manifests/server.pp

@@ -50,7 +50,7 @@ class subunit2sql::server (
   }
 
   exec { 'upgrade_subunit2sql_db':
-    command     => 'subunit2sql-db-manage --config-file /etc/subunit2sql.conf upgrade head',
+    command     => 'subunit2sql-db-manage --config-file /etc/subunit2sql.conf upgrade head &> /var/log/subunit2sql_migration.log &',
     path        => '/usr/local/bin:/usr/bin:/bin/',
     subscribe   => Exec['backup_subunit2sql_db'],
     refreshonly => true,