Check for service checks on local host

[widhalmt]

Check if there are sufficient checks checking the local (meaning: Icinga) host.

Since we can not safely determine if a specific metric is checked (there are far too many plugins for the same task) we could just count how many services the local host has. If this count is below, say, 5, then give the suggestion to review the checks including a link to the official documentation about monitoring monitoring hosts.