Group services depending on the host they are related to.

This will ease the setting of downtimes for services related to a host,
in particular for the external check such as the HTTP ones.

For that we add a custom variable to this kind of remote checks which
contains the concerned host name. We also define a servicegroup named as
this host. We then assign services to this group using a custom Icinga2
function.

Refs: #11277