Added service excludes feature

https://github.com/naparuba/shinken/pull/981 added the ability to override a service properties from host configuration.

This patch adds the complementary ability to exclude services from a host. This is mostly useful for services inherited from packs or built from hostgroup expansion where identified hosts should no be defined some of the services.

This avoids having to define intermediary templates or to redefine services for hosts that are *almost* consistent with others.

Example:

Let's imagine a set of linux servers. All of them are using the `linux` pack, but one of them should not be checked its time drift. This could be simply written:

    define host {
        use              linux,...
        host_name        host-without-time
        service_excludes Time
        ...
    }