Option to deactivate modules

[gracinet]

In some case, it's useful to deactivate some modules before launching tests or doing upgrades.

For instance, we have currently a project that depends on ldap_synchronization, and the upgrade script can't run without a properly configured LDAP server. Of course it'd be even better to setup a full integration testing harness, but we don't want to depend too hard on that, especially since in any case it'd need a modification of that module.