Add "destination" to Hostmaster alias so "drush dl module" commands put the module in the right folder.

When downloading modules to extend Aegir hostmaster, typically users would use "drush dl" command.

By default, this sets the destination to "sites/all/modules".

However, when upgrading, the platform is rebuilt, so hostmaster modules in sites/all/modules are lost. Most users don't know this, it never warns them.

I tried, and if you add "destination" => "sites/hostmaster.com/modules" to the hostmaster alias, drush dl commands automatically go there.

Let's add this to the hostmaster alias so it's a little easier to extend.