Allow local.settings.php to override global.inc to make it usable

As Miguel pointed out in our IRC conversation earlier today, the order of local/global includes in the settings.php template should be reversed to make it really usable.

Currently the local include will work only for settings not defined/hardcoded in global.inc, but what if we want to enforce standard caching for all sites but still want to enforce aggressive (or none) caching for one or two sites? Currently we need to use not really good extra if/else for exceptions in global.inc, so why not to allow local include to do that where it is really needed?

Of course we assume that both global and local include files are managed by server admin and not by a site admin, so there is no reason to be afraid because the global.inc no longer can control everything (having the last word for all sites).

The commit: http://gitorious.org/aegir/provision/commit/db0905d927f5943a97c3143e861f...

The patch attached.