icinga: check its own configuration
At the moment, whenever one puppet change breaks the icinga config we
get exactly one puppet failure from neon, which then recovers (as the
icinga config will be reloaded just once), but it seems we can't notice
it promptly. The net effect is that the icinga config is often broken
and it requires me and others to fix it a posteriori when we're doing
Introducting an icinga check on its own configuration should allow us to
notice the problem more explicitly, and persistently.