Fix dependency problems with APT repo handling

The first issue was that `Exec[apt_get_update_for_nginx]` resource is only needed if `$manage_repo == true`, so it was put inside the corresponding if block.

But even inside the if-block, `Exec[apt_get_update_for_nginx]` could fail when `$package_source` has an invalid value and no `Apt::Source[nginx]` is defined, so the respective `Apt::Source[nginx]`, if defined, will now notify `Exec[apt_get_update_for_nginx]` and `Exec[apt_get_update_for_nginx]` will no longer subscribe to `Apt::Source[nginx]`.

It would probably make sense to add a call to fail() in the default case of `$package_source`.